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行政院國家科學委員會專題研究計畫 成果報告

高分子溶液之物理凝膠化過程中相變耦合機理

計畫類別: 個別型計畫

計畫編號: NSC93-2216-E-011-013-

執行期間: 93 年 08 月 01 日至 94 年 07 月 31 日 執行單位: 國立臺灣科技大學高分子工程系

計畫主持人: 洪伯達

報告類型: 精簡報告

處理方式: 本計畫可公開查詢

中 華 民 國 94 年 10 月 28 日

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行政院國家科學委員會補助專題研究計畫 ■ 成 果 報 告

□期中進度報告 高分子溶液之物理凝膠化過程中相變耦合機理

計畫類別:■ 個別型計畫 □ 整合型計畫 計畫編號:NSC 93-2216-E-011-013

執行期間:2004 年 08 月 01 日至 2005 年 07 月 31 日

計畫主持人:洪 伯 達 共同主持人:

計畫參與人員:

成果報告類型(依經費核定清單規定繳交):■精簡報告 □完整報告

本成果報告包括以下應繳交之附件:

□赴國外出差或研習心得報告一份

□赴大陸地區出差或研習心得報告一份

□出席國際學術會議心得報告及發表之論文各一份

□國際合作研究計畫國外研究報告書一份

處理方式:除產學合作研究計畫、提升產業技術及人才培育研究計畫、列 管計畫及下列情形者外,得立即公開查詢

□涉及專利或其他智慧財產權,□一年□二年後可公開查詢

執行單位:國立台灣科技大學 高分子工程系

中 華 民 國 94 年 10 月 27 日

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一,研究計畫中文摘要:

關鍵詞:高分子溶液;凝膠化;相變;光散射。

Small-Angle Light Scattering(SALS)是研究膠體聚集、高分子摻合物、因此設計一台能同時滿足real-time 的追 蹤結構形成及涵蓋結構發展的特徵長度尺度範圍的光散射儀,就是本研究室四年來的重點設備項目。歷經兩個 國科會計畫案三年的經費補助之下,本研究室成功設計與組裝台灣第一台高解析度Time-Resolved Small-Angle Light Scattering(TRSALS)。另外,有關高分子物理凝膠結構的形成,本研究室歷經了長期且有系統的研究。

有別於一般研究學者將主軸置於凝膠物性與應用的研究,我們將研究的核心放在『凝膠化』這一個行為上。本 研究室嘗試將凝膠化與相變化做一連結,並指出高分子物理凝膠形成基本上涉及三種相變行為:1.高分子結晶 化(液-固相轉變);2.高分子溶液的相分離(nucleation and growth or spinodal decomposition最為重要);

3.Percolation 現象(幾何相變化)。因此由相變化的觀點來看,凝膠化行為無疑是複雜的。因為它不但包含了 多種相變的耦合,同時也涉及了微觀結構的形成(指高分子鏈間微結晶的形成,高分子溶液的相分離)與巨觀 物理狀態的改變(包括percolation 現象與凝膠彈性)。而在這兩種尺度之間,又牽涉到自組織結構(self-assembling processes)的形成與演化的複雜行為。有關凝膠結構形成的相關研究,在目前似乎達到的瓶頸。近年來出色的 論文並不多,應用性的研究幾乎成為近年來研究的主流。本研究室長期進行凝膠結構的研究,深刻瞭解到此一 問題的嚴重性。因此高解析度TRSALS 的成功架設,堪稱本研究室近幾年來最大的成果。本儀器不僅能對凝膠 乃至於其他諸如高分子摻合物、綜述本計畫的核心有下面幾個方向:1.結晶性(光學異方性)微凝膠的結構本 質及其熱力學驅動力為何?是結晶成核誘導局域的濃度漲落(concentration fluctuation),還是相反過程。不論 是何者,他都應該有其對應的熱力學相圖來具體描述驅動力的來源。2.孤立的微凝膠粒子是如何相互聚集形成 三次元的網目構造,即凝膠化,聚集動力學為何?這部分牽涉到『非平衡熱力學』的部分,因此自組織結構

(self-assembly)的形成、成長以及標度(scaling)將是這部分的主題。3 建立正確的散射結構因子,給出正確 的凝膠結構模型。

研究計畫英文摘要:

Keywords: Polymer solution; Physical gelation; Phase transition;

Light Scattering

In this project, the coupling mechanism of phase transitions in structural formation of polymer physical gels will be deeply discussed through the analyses on time resolved small angle light scattering (TRSALS) data. In our previous studies, we have reported the first real-time observation of crystal nucleation and concentration fluctuation during the physical gelation process of a crystalline polymer. It also implies that the spontaneous concentration fluctuation by spinodal decomposition is not a prerequisite for the formation of large-scale heterogeneous gels. Our previous results showed that the gelation process could be governed by the coupling of several phase transitions such as crystallization, liquid-liquid phase separation and percolation. Viewed in this light, we would question whether the formation of the birefringent droplet can be compared to the non-classical “spinodal” nucleation. However, there is need for further investigation on this question. At later stage of gelation, the hard-sphere-like colloid aggregation dominates the gelation process. The key idea for aggregation in colloidal suspensions would be demonstrated by its fractal nature, and the gelation may be considered as a direct consequence of the growth of fractal structures. According to the sticking probability, two limiting regimes of the aggregation process have been identified, namely, diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA). However, there is not enough evidence to support the fractal aggregation of the birefringent droplets in our previous work. We will try to find evidence for supporting this proposal through this project.

二,報告內容

[第一部份] Gelation Behavior and Structural Transition in PLLA/NMP Solutions

ABSTRACT

In this study, we first investigate the property of dilute PLLA/NMP solutions in order to clarify the molecular interaction between PLLA and NMP. The Huggins constant KH=0.29 indicated that the solvent used of NMP is a good solvent to PLLA. From the analysis of the gelation kinetics at 30℃, the gelation mechanism is not dominated by percolation transition but bimolecular association with a nucleation or reaction rate-determining step. On the other hand, the gelation at high concentrations above 8.4g/dl, the crystal formation of PLLA is clearly observed. The structural formation during gelation is probed by time-resolved small angle light scattering measurements. A unique change in the scattering pattern is found during gelation. At the initial stage of the gelation, an X-type Hv scattering pattern is detected, indicating that the rod-like structure is formed ant its size increases with time evolution. At the late stage of gelation, the Hv scattering pattern changes to be a four-leaf clover type i.e. a spherulite-like structure. The morphological transition from rod-like structure to spherulite-like structure is first time observer during physical gelation of polymer solutions. It may be expected that the gelation is characteristic of kinetic view of point through the analysis of the change in scattering intensity with gelation time and various concentration of the solutions.

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Keywords: gelation kinetics, SALS, rod-like, spherulite-like structure Introduction

Poly(lactic cid)(PLLA),(PDLLA),(PDLA) are well established as very useful biodegradable polymers1, covering a wide range application, such as dental, drug deliver, orthopedic which have emerged from the polymer materials. In former study, De Santis and Kovacs reported that the PLLA have right-handed helical crystalline structure2, in addition to forming gel from PLLA/NMP solution, the mechanisms of structure formation for biopolymer gels are very complex. It’s well know that many factors effect the gelation kinetic process, such as concentration, temperature, molecular weight, and the type of solvent used. We choose poly (L-lactic acid) which is a semi crystalline polymer depended on its degree of stereoregularity3 and easy to gelation. The first point to notice is the degree of the polymer-solvent interaction. In previous study, we reported that the physical properties of PVC gels which strongly dependent on the aggregation degree of polymer chains related to the solvent types. In addition to this, PVA/EG, PVA/NMP solutions showed that the melting temperature and crystalline of PVA/EG gel were higher than those of PVA/NMP gels. These results give us consider that the polymer-solvent interaction must be play an important role on the aggregation behavior for polymer chains during the gelation of the solution. Physical gel is a three-dimensional of polymer chain which cross-linked by physical junction point, and it also can be arise by specific physical influence i.e., first-order phase transition (crystallization and liquid-liquid phase transition phase separation), some special molecular association (polar, complex, or colloid). In our present study, the gelation behavior has been discussed; we confine our attention to the case of gels associated with phase transition. Physical gels passing through several kinds of phase transition are complicated systems, and it is typically distinguish from the coupling of several phase transition like the liquid –liquid phase separation, the crystalline formation of the polymer chain segments, and percolation phenomena. Our concern is to consider the gelation mechanism of PLLA/NMP solution and to interference the reasonable theory for the structure to develop.

TRSALS (Time-resolved small angel light scattering) is a powerful tool for study microstructure in polymers. We can find reciprocal-space (Fourier-space) of images to recover the real-time (Physical-space) information. As present result, the value n of kinetics of gelation=2 which is bimolecular association with a nucleation or reaction rate-determining step, and we can concentrate on small-angle light scattering technique combining Hv, Vv scattering discuss the forming of physical gels. The purpose here is to explore a little further into the structure develop of PLLA/NMP solution through gelation behavior and to clarify the relationship of mesoscopic structure. Relatively little is known about the behavior of rod-like polymers in good solvents, we first time discover the morphological transition from rod-like structure to spherulite-like structure during physical gelation of polymer solutions. The scattering from spherulite which are most commonly observed in poly-α-olefins, such as polyethylene and polypropylene has been Stein and coworkers. The clear four-leaf clover Hv scattering patterns can be discover after the X-type Hv scattering patterns in our system. There are many cases where Hashimoto.., have already discussed rod-like collagen films, PTFE, cellulose derivatives and etc. We use rod-like scattering model A4-6 from Hashimoto which the optical axis is assumed to form a polar angle ω0 with the vector r1 and to lie in a particular plane specified by an azimuthal angle γ. By using random assembly of anisotropic rods in three-dimensional space mode and Stein-Rhodes equation, we figure out the average rods length link together with spherulite radius have a semi-linearity relationship. In contrast with previous study, the structure evolve during the gelation have a specific behavior, and the time between the different kinds of structure have transitional regime which rods change into spherulites.

There is further evidence to suggest that morphology changes by Optical Microscopy graph and then combine above result we present the real-time measurement during the gelation process of a polymer solution.

Experimental Sections

Materials. The Biodegradable polymer used in this study is poly (L-lactic acid) (PLLA) pellet (Mw=3.00×105 and i.v. =4.00-5.20, Polysciences, Inc.) The solvent,1-Methy-2-pyrrolidinone (NMP), (anhydrous, 99.5﹪,water﹤0.005﹪, Aldrich Chemical Co. Ltd, USA) was used in this work. The PLLA solutions were prepared in a precleaned wide mouth bottle, with stirring at 160℃ for 1 hr until they dissolved into homogeneous solutions. The PLLA solutions with concentrations 0.05-12gL-1, then cooled into a thermostat oven at constant temperature 30℃ for one day to stabilize the solutions before the measurements.

Gelation Rate. First, PLLA solution with various concentration containing in sealed test tube (8mm i.d. and 10cm in height) were kept in an oven at 160℃ for about 1h to make the solutions homogeneous before measurements. Then the hot solutions were quickly transferred into water bath being kept at a given temperature to be controlled within ±0.1℃. The test tube tilting method was used for determining the gelation time (tgel), which was defined by observing cessation of the liquid flow inside the test tube when it was titled, and the gelation time was monitored just after the test tube was put into the thermostatic bath. The reciprocal of gelation time of the solution is defined as the apparent gelation rate, tgel-1.

Intrinsic Viscosity. The determination of the viscosity of dilute PLLA solution was carried out using an Ubbelohde viscometer immersed in a thermostatic bath to be held at 30 ±0.1℃ for 1 h. The intrinsic viscosity, [η], was obtained by the Huggins equation7

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[ ] K [ ] C C

C t

t t

H

sp 2

0

0 η η η

+

=

= (1)

where t is the flow time of the dilute solution, t0 the flow time of the pure solvent, C the concentration of polymer, ηsp specific viscosity, and KH the Huggins constant.

Phase-contrast and Polarized Light Microscope PLLA/NMP solution prepared by two cover slip (gap between <30um) and held temperature with a Linkam THMS 600 hot stage from160℃ to 30 ±0.1℃. Light microscopy was done with a Leica DMLP microscope in transmission equipped with a magnifier 2×, Leica 10× (N PLAN) and 20× (C PLAN, long free working distance) phase contrast objectives and magnification changer. A MOTIC MC2000 camera (2.0 Mega pixels CMOS camera) was attached to the microscope and coupled to a desktop computer.

TRSALS Measurement

The solution specimen was inserted into a hot stage (TMS-600 heating stage Linkam Scientific Co.) at a given the same of gelation kinetics temperature 30 ±0.1℃. A vertical -polarized laser beam (5mW He-New laser having a wavelength of 632.8 nm) was used as the incident source and the polarization direction of the beam was varied by a half-wave plate. The scattered light intensity of the sample was directly imaged through a Fourier lens and an analyzer onto the CCD camera (Apogee Instruments Inc., Apogee Alta U2000 with 1600×1200 array, 7.4 x 7.4 micron pixels). The digitized images were transferred the real-time processing to a personal computer. In our design the angular range for which reliable data can be collect is about θ=0.5o-20o, corresponding to q

=0.127-5.059 um-1 and length scale 1.24-49.45 um[q=(4πm1/λ) sin(θ/2), where q is the scattering vector, λ is the wavelength of incident light, and m1 is the refractive index of an isotropic medium].

Result and Discussion

Kinetics of PLLA/NMP Gelation

Figure 1 shows the viscosities of PLLA/NMP diluted solutions as a function of polymer concentration. Using classic Huggins and Kramer equations, the obtained intrinsic viscosity [η] is 2.4 dl/g and Huggins constant KH is 0.29 for PLLA/NMP solutions. Generally, the value of Huggins constant, KH, could also be used to predict the degree of polymer-solvent interaction. In θ solvent (KH=0.52), the polymer chains exhibit unperturbed coils. In a good solvent (KH<0.52), the polymer chains should exhibit relatively extend conformations, and in poor solvent (0.8<KH<1.3), the polymer chains collapse and the intermolecular aggregation occurs easily. Consequently, the PLLA/NMP solution with lower (KH=0.29) value is considered to be a good solvent, which means that a higher solubility and strong attractive interaction between the polymer and solvent. The intrinsic viscosity, [η], is a characteristic function for single polymer chain in solution, depending much on the molar mass, the structure and conformation of the polymer, the polymer-solvent interaction and the temperature. Furthermore, the [η] value dimension is volume per unit mass so it can be appropriately reflected by the effective hydrodynamic volume of polymer chain in solution.

On the other hand, the chain overlapping concentration C* can be calculated by the reciprocal of the intrinsic viscosity, (1/η). The C* value is about 0.42gL-1 for the PLLA/NMP solution at 30℃ in this work. When the polymer solution concentration is higher than C*, infinite junction points are formed over the whole solution; polymer chains should be overlapped or entangled, while the polymer concentration of the solution is lower then C*, which exhibits homogenous polymer solution. This result might be considered owing to fact that good affinity of the solvent used might be more relatively spread of PLLA chains.

It is important to understand the kinetics of gelation behavior for study the mechanisms of gelation, and it is usually done through the analysis of the apparent of gelation rate. The apparent gelation rate is obtained from the reciprocal of gelation time, tgel, which is the time required for the polymer solution to form the gel. Figure 2 shows the tgel-1 values of PLLA/NMP solutions as a function of polymer concentration at 30℃. The result shows that the tgel-1 increase with increasing concentration rapidly in the short regime. In order to determined the critical concentration for gelation, C*gel, which is an important parameter to understand the mechanism of gelation, and rate curve in Figure 2. were extrapolated to zero gelation rate. The C*gel values at 30℃ is about 8.4gdL-1, and we would like to express the gelation late as a function of concentration. In previous study8-9, we have already used three different kinds of kinetics gelation process for Table 1.

Table 1. The different n values of kinetics of gelation

To provide a general description of the concentration function, the reduced concentration and the exponent n should be introduced. Therefore, at a given temperature the gelation rate is express as a general relationship by10-14

The value n of kinetics of gelation n=0.45 Percolation model

n=1 Diffusion Control model n=2 Bimolecular association

n

gel gel

gel C

C t C

⎡ −

*

*

1

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(2)

Where n is exponent value depending on the gelation mechanism. Figure 3. shows double logarithmic of tgel-1 and reduced concentration at 30℃. We would like to emphasize is n value, determined by the slope of the concentration region. The point is that n value can explain the kinetics of PLLA/NMP gelation process. From the slope it can be seen that the value n is 2 for bimolecular association in the cross-link process which very similar to our previous study for gelation of PVDF/TG solutions in high concentration, not the percolation exponent “0.45’’ in a three dimensional lattice model or the exponent “1’’ for the diffusion of polymer chains in the phase separation process.

In our previously study, we have already determined the four regions at polymer solution as following in Figure 413. In the finite dilution limit, which is defined as a concentration below [η]

C~1, the polymer chain acts as an isolated coil. If the concentration of polymer in a solution is raised, the hydrodynamic screening limit will be reached, and the relative proximity of neighboring chains allows polymer-polymer intermolecular interactions to influence the motion of polymer chains. Perturbation of the polymer motion by this mechanism is expected to occur above a concentration as defined by [η] C>1. The effect may be expected to be cumulative up to the concentration corresponding to the chain overlapping concentration, where the close packing of polymer coils in solution exists at the concentration [η]C~4. Once the overlapping concentration is attained, the polymer motion will be dominated by the present of the polymer-polymer interactions.

If the increasing concentration rise beyond the limit of [η] C~10, the interpenetration of the polymer coils or the pseudo matrix-gel will be formed. But in Figure 4, the critical point of gelation concentration is at 20 after [η] C =10. The result is different from PVC/BrBz gelation in solution, the polymer coils of PLLA in the concentration region have already interpenetrated with each other.

And the critical behavior is happened very late after the coil-coil interpenetration. In the last few years, our articles have been devoted to the study of PVC/ClBz gel which may occur in a heterogeneous at hydrodynamic screening region contributed by the liquid-liquid separation or spinodal decomposition and not same in this work. We suppose that the initial gelation behavior isn’t controlled by the chain overlapping process, all effect by the front view point of PLLA/NMP interaction in dilute solution. The larger intrinsic viscosity reflects the extender polymer chain in the solution. In the other words, we can find the critical gelation point at higher concentration region.

Scattering Patterns of the PLLA/NMP gel structure transition

Figure 5. Shows the time evolution of Hv (left) and Vv (right) scattering patterns from the isothermal gelation for 8.8gdL-1 PLLA/NMP solution at 30℃. At first appearance from original data, we find out particular scattering pattern from 2600sec to 3500sec. The results show that Hv scattering patterns have an obvious transition which from the initial stage of the gelation, an X-type Hv scattering pattern is detected10-11, indicating that the rod-like structure is formed ant its size increases with time evolution. At the late stage of gelation, the Hv scattering pattern changes to be a four-leaf clover type i.e. a spherulite-like structure. We would like to divide into three regimes: In (A) (2600-3000sec), an X-type scattering patterns with a monotonic decay of the scattering intensity, which at the center of the pattern appears the same as the scattering by an anisotropic rod structure4-6, (B) (3000-3300sec), a complex-type scattering patterns with sharp slope intensity distribution , which at the middle of scattering patterns produce qm arises the semi-spherulite-like structure, and (C) (3300-3500sec), in the two diagonal directions appears to have fourfold symmetrical pattern at high scattering angles with the maximum scattering intensity. Little attention has been given to the point that the kinetics of gelation has specific structure transition.

It is interesting that the time changes in the shape of the Vv scattering patterns also have obvious variation. Before 3000sec, we see only a monotonic decay of the scattering intensity with θ and a nearly circularly symmetrical Vv scattering pattern. After 3000sec, the shape of dumbbells appears and by the time changes the scattering patterns becomes clearer follow spherulite texture growth. We have to indicate that the initial pattern with X-type implies the growth of the anisotropic rods and between the conversions of the pattern from cross shape to four-leaf-clover shape with the emergence of transition stage are the main characteristics of the rod-like to spherulite-like transition.

In figure 6. By using optical microscopy we catch the structure of PLLA/NMP solution gelation behavior. First, figure 6. (A), in order to differentiate local structure of the gelation course, we use phase-contrast to emphasize the image of growth in 2D. To begin with initial image present evident rod shape and then become bundle shape, finally the form get into a spheroidal shape. From the figure 6.(b) Polarized Light Process, it is clear that PLLA gel exhibits a spherulite morphology at the final of gelation structure and in the initial solution graph which appear slight rod structure which can proof that its have anisotropic texture.

Rod-like scattering patterns were first given by Rhodes and Stein15, and Hashimoto also has been discussinga distribution of isotropic rods in two or three dimension space. The Hv scattering pattern in Rod-like systems present X-type or +-type which differ from the spherulitic scattering in that the rod-like scattering generally does not exhibit distinct scattering maxima as Imax and qmax

and appear monotonic decay Intensity distribution. By using the model-A4-6,16 of anisotropic rod-like particles for which the optical axis is assumed to form a polar ω0 with the vector r1 and to

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lie in a particular plane specified by an azimuthal angle γ. The theory considers the difference between relative HV scattering intensities at u=0o and u=45o, i.e., for model-A rods:

) 3 30 35 )(

(cos )

( )

(IHV u=45o IHV u=0o =K5'P4 ω0 A B+ C (3)

The relative intensity distribution passes through a maximum at scattering angle θmax given by

2 ) 2 sin(

80 .

4 max

max

θ λ πL

U = = (4)

From which the length of the rod, “L”, may be determined. It should be noted, however, that this method utilizes intensity distributions at smaller scattering angle than the method proposed in this work. Since intensity distributions at small angles are most sensitive to interparticle interference, the letter method is expected to be the better. The characteristics of the rod-like scattering as compared with that of the scattering from a spherulitic crystalline superstructure were manifested in that the intensity of the rod-like scattering continuously decease with increasing scattering angle, while that of the spherulitic scattering passes through maximum at a scattering angle reciprocally related to an average size of spherulite. The average size of the spherulites can be evaluated easily and rapidly by observing θmax, the scattering angle at which the Hv spherulitic scattering intensity become maximum (see the below left of Figure 5. 3500sec Hv scattering pattern) and by applying the equation given by Stein-Rhodes,

2 ) 4 sin(

10 .

4 max

max

θ λ

πR

U = = (5)

Where R is the radius of average size of the spherulite assumed to be spherical, and λ is the wavelength of light in a medium. It well is clear from these expressions for us to calculate the size of the structure by SALS. To study the growth kinetics, the quantitative analysis of the average rod length and spherulite radius was given as follows. Below Figure 7 shows the variation in structural parameters of 8.8gdL-1 PLLA/NMP solution with gelation time. In the figure, we would like to divide into four regimes: The first regime is the change of the rod length shows a linear growth behavior, and then into Regime II, we used the model-A rod formulation and Stein’s Spherulite form to fit the complex-regime which finds out fast growth rate. We assume the rods transit to bundle structure and fixed in the space with the specific growth rate. Then, in regime III the spherulite-like structure is forming in the three dimension networks which growth rate is also appear linear relationship. Finally, at longer time in Regime VI, the spherulite growth rate was restrained by the gelation behavior after tgel. Combing above result, we established the structural transition model on the top of Figure 7. In conclusion, (1) By the kinetics of PLLA/NMP gelation, we get characteristic value is 2 just conform with the structure detect from SALS which form by bimolecular association with a nucleation or reaction rate-determining step. (2) Investigating the growth of structure parameter, the rod-like to spherulite transition process effected by the gelation behavior in the mode we established.

References

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3. Thomas H, Barrows, Ph.D. Synthetic Bioabsorbable Polymers

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7. Spering LH. Introduction to Physical Polymer science, New York: Wiley, 1986.

8. Hong, P. D.; Chou, C. M.; Polymer 2000:41:8311.

9. Hong, P. D.; Chou, C. M.; Macromolecules 2000:33:9673.

10. Chou, C. M. ; Hong, P. D.; Macromolecules 2003:36:7331.

11. Chou, C. M. ; Hong, P. D.; Macromolecules 2004:37:5596.

12. Mal, S.; Maiti, P.; Nandi. K. Macromolecules 1995:28:2371.

13. Dikshit, A. K.; Nandi. A. K. Macromolecules 1998:31:8886.

14. Ohkura, M.; Kanaya, T.; Kaji, K. Polymer 1992:33:5044.

15. Rhodees MB, Stein RS. J. Polym. Sci. Paet A-2, 1969:7: 1539.

16. Frish H. L.;Simha, R. In Reology Theory and Applications; Erich, F. R., Ed,; Academic Press:

New York, 195

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[第二部份] Structural Formation and Coloration Mechanism of iso-Poly(1-butene) /o-Xylene Physical Gel

ABSTRACT

Poly(1-butene) gels in some solvents of benzene-derivatives show a particular coloration phenomenon. Yet the structural formation of this phenomenon has not been defined clearly. Therefore, we carried out the study by phase contrast microscope (PCM) and time-resolved small angle light scattering (TRSALS) to determine the gelation process of isotatic-Poly(1-butene)/o-Xylene solution (iPB/o-xylene). In contrast with the classic spinodal gel, a novel aspect on nucleation gel is more suitable to describe our experimental results. Namely, the process of structural development from the homogeneous iPB/o-xylene solution is confirmed by the nucleation and the growth of the microgels, the diffusive aggregation of the microgels, and the percolation in cluster-cluster aggregation process. In addition, the appearance of the solution changes from transparent stage through cloudy stage to coloration stage. We can conclude that the change of refractive index between polymer microgels and the solvent is the reason why the coloration phenomenon develops in the Poly (1-butene) gels.

Keywords: coloration, nucleation gel, fractal aggregation Introduction

Gels represent a unique state of matter characterized by space-spanning network ramified structures responsible for soild like properties of macroscopic. Recent small-angle light scattering experiments have revealed that diffusively aggregating spherical particles develop structure on a meso-scopic length scale. But most of the studies[1] have focus on to the colloidal gelation process.

Very few attempts have been made at polymer gelation. Among all the polymer gelation process, polmer physical gel, physical junctions can arise either as a result of a phase transition or through some specific molecular association or as a result of entanglement, is the most complicated. At the beginning of our study[2], we focused on the structural properties of the junction point gel by wide angle static light scattering, X-ray, etc. However, gelation behavior is a large scale first order phase transition. We should take the small angle light scattering to overview the structural formation of the physical gel.

We report the real-time observation of the structural development by small angle light scattering[3,4]. Experimentally Observed the spinodal decomposition[5] is not a prerequisite for the formation of large-scale heterogeneous gels. A novel type nucleation gel structure model has been developed. Unlike a typical spinodal gel, a novel nucleation gel, is developed by a complete model presently.

The isotactic polypropylene and poly(1-butene) in some organic solvents develop colors[6,7], although polypropylene and poly(1-butene) themselves are colorless. We also indicated that the coloration phenomenon is due to selective light scattering from size-controlled polymer gel networks with solvent, having the same refractive index in the visible wavelength region.

In order to clarify the structural formation and coloration mechanism of iso-Poly(1-butene)/o-Xylene gel. In this work we establish a simple model By SALS and PCM. The model is quite similar with the nucleation gel. However in our experiment range the ripening process are not observed in the late stage after the gelation. Further, coloration phenomenon of the solution may be explained as the refractive index of the solution changes to the visible wavelength region during the aggregation process of the microgels.

Experimental Sections

Materials Both Isotactic Poly(1-butene) (iPB) (Mw = 1.85×105) and the solvent o-Xylene in this work are from Aldrich Chem. Company. All the samples prepared in the experiment are heating and stirring at 413K to make the iPB solution (16.5 g/dL) homogeneous and then quench by liquid nitrogen to gelation temperature 303K

Small-Angle Light Scattering The SALS schermatic diagram has described previously[3]. The incident source is a plane-polarized laser beam(5 mW) He-Ne laser(632.8 nm). The experiment samples placed on a heating stage(Linkam Scientific Co. TMS-600). The scattered light intensity of the sample was directly image through a Fouier lens and an analyzer onto a CCD camera (Apogee Instruments Inc., Apogee Alta U2000 with 1600×1200 array, 7.4 ×7.4 pixels).The angular regular range is about θ=0.5~20°, corresponding to q =0.130~5.17μm-1 and length scale 1.27~48.3μm Optical Microscopy (Phase-Contrast Microscopy) Optical microscopy was done with a Leica DMLP microscope in transmission equipped with Leica 10× (N PLAN) and 20× (C PLAN, long free working distance) phase contrast objectives and magnification changer. A MOTIC MC-2000 camera (2.0 Megapixels CMOS camera) was attached to the microscope and coupled to a desktop computer.

Results and Discussion Direct Observation

We describe direct observation of the samples. A Sample sealed in a test tube keep at 413K for

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30min to insure the solution homogeneous and place into a 303k undisturbed water bath. The photographs of color change in the gelation process Fig.1 (a) homogeneous sol state (b) after the gelation. Obviously, the solution changes from transparent stage through cloudy stage to coloration stage. The gelation completes in a few hours. In our previously work, we indicated the coloration phenomenon may impute to selective light scattering from size-controlled polymer networks with solvent, having the same refractive index in the visible wavelength region. Additionally, the SEM images show the equilibrium structure for polypropylene gel in o-xylene system.[8] It seems that the gel network formation is from aggregation with the mirogels. However this phenomenon is interested us to discuss what happened during the structural formation of the gel.

Small-angle Light Scattering and Phase-contrast Microscope

First, we carried out our light-scattering experiment. Figure 2 shows a series scattering patterns Hv (top) and Vv (middle) evolution with time during isothermal gelation. The time changes with the Hv scattering patterns can be divided into two stage: (1) a four-leaf-clover pattern in early stage (t<4000s), the peak intensity rises gradually and the peak position shift towards a smaller q-range (2) a four-leaf-clover pattern in late stage (t>4000s), the peak intensity continuous increase but the peak position hold on at a fix q-range. In early stage, when an oversaturated crystalline polymer solution is quenched from homogeneous phase to a meta-stable region, the nucleation and growth process can govern by coupling of the local concentration fluctuation and polymer crystallization. Thus the four-leaf-clover pattern can be interpreted as the growth of the anisotropic crystal with time. In contrast with early stage, late stage we only observe the scattering intensity increase with time. It can be explained why the nuclei growth to reach a critical size and the aggregation behavior occur.

On the other hand, Figure 3 shows the time evolution of the Vv scattering profiles the at azimuthal angle ψ=45.º The time changes with the Vv scattering patterns, we only a monotonic decay of the scattering intensity with scattering angle θ. The intensity increase with time gradually.

Beside, the Vv scattering also shows the influence of the orientation order parameter on the scattering pattern: (1) In the early stage, the scattering pattern along the equator is significantly stronger than along the meridian. Nakai et al.[9] also show a similar scattering pattern in the Vv pattern in their study.(2) It changes to a nearly circularly symmetrical scattering pattern in the late stage.

Second, support for above picture is obtained by observing the gelation process under the microscope. A time course of the iPB/o-xylene pictures by PCM shows in the Figure 2 bottom.

Experimentally observed discontinuous droplet form from the homogeneous state and aggregate formation three dimension network structure. Let us examine the mechanism of this structural formation in more detail. In the beginning (t <1000s), we only see a few droplets formation and suspension in the solution. Following (1000s < t < 3000s), the droplets radius increase with time and some droplets begin to collide and aggregate to each other. Then nearly all the droplets fix at the same radius and random aggregate to become large scale clusters (3000s < t < 6000s).

Ultimately, after 6000s the cluster-cluster continuous aggregate to form a three dimension network.

In order to justify the network we specifically scale up the last picture (the radius of a droplet is about 3-4μm). It is clear that the iPB gel exhibit a spherical morphology and the spheres or rather the microgels are connected with each other to form the structure. The similar spherical morphology of iPB gel has been observed previously.

Combining the results of the SALS and PCM experiment finally, It can sum up that the process of structural development from the homogeneous iPB/o-xylene solution is through the nucleation and the growth of the microgels, the diffusive aggregation of the microgels, and the percolation in cluster-cluster aggregation process.

Fractal Nature of the Structure

According to the last section, the iPB gel formation can be considered as the fractal aggregation of the isolated microgels. For a fractal aggregate with no multiple scattering, the angular scattering can be expressed by

( )q F( )q S(q)

Iscatt

WhereF( )q is the Form factor,S(q)is the Structure factor,andq is the scattering vector.

In the range of fractal geometry,S(q)is a power law of the form

q df

q

S( ) when rc<< /1 q<<Rg

Where rcis the particle radius and the Rgis the radius of gyration of a aggregation.

This result is often used to determine the fractal dimension of a fractal object. Figure 4 (a) shows the double logarithmic plot of Ivv(q)versusq at sol stage (t=250s) and fractal aggregation stage (t=4000s). In sol state (t=250s), the single-particle form factor at lowq were practically constant over the relative rangeq .The scattered intensityIvv( )q can therefore be taken to represent the structure factor,S(q). In fractal aggregation stage (t=4000s), the fractal dimension dfcalculated from the equation is 1.61. In the same way Figure 4 (b) shows the plot of calculation df values versus time. The fractal dimensiondf<1 stand for a topological dimension dT =0, since it must be

t

f d

d > . Therefore, at the beginning of the df<1 regime are not connected. At longer times, the df

value rises, goes past the unitary value, and finally approach to the stationary value df≒1.7. The fractal dimension stops growing at the time of macroscope gelation.[5,11] By compare with the

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results in the PCM image and light scattering data, diffusion –limit colloid aggregation (DLCA) model is more suitable to describe in our system.

DLCA process for the gel structural formation

Understanding aggregation phenomena is fundamental in the gel structure. Two limiting regimes of the irreversible aggregation process have been identified. In the former case, named diffusion-limited colloid aggregation (DLCA), this rapid aggregation is only limited by time taken for the cluster to meet each other by diffusion. In the latter, termed reaction-limited colloid aggregation (RLCA), the slow aggregation rate is limited by time taken for two clusters to overcome a certain potential barrier between the sticking particles, by a thermal fluctuation. It is well know that in the DLCA model the fractal dimension is about 1.7 and in the RLCA model the fractal dimension is close to 2.1. As shows in Figure 5, we draw a schematic representation of the DLCA process from PLM real space image.

Conclusions

(1) From the analysis of the time resolved small angle light scattering Hv and Vv patterns during the gelation process, nucleation behavior and growth in the early stage and aggregation with the mircgels in the solution have been observed

(2) A serious phase-contrast microscope pictures show that discontinuous droplet nuclei from the homogeneous solution, aggregate formation the large scale cluster, and the percolation in cluster-cluster aggregation process form three dimension network structure

(3) The fractal Dimension d value rises, goes past the unitary value, and finally approach to the f stationary value df≒1.7.It is well known the DLCA aggregation model

(4) We establish a simple model of the iPB/o-xylne microgels DLCA aggregation from PLM (5) Coloration phenomenon of the solution can be explained as the refractive index of the solution

changes to the visible wavelength region during the microgels aggregation process.

In this work we establish a simple model By SALS and PCM. The model is quite similar with the nucleation gel. However in our experiment range the ripening process are not observed in the late stage after the gelation. Further, coloration mechanism of the solution can results from the aggregation phenomenon.

References

1. W. C. K. Poon, A. D. Pirie, and P. N. Pusey Faraday Discuss., 101, 65 (1995) 2. P. D. Hong, C. M. Chou Polymer, 41, 8311 (2000)

3. C. M. Chou, P. D. Hong, Macromolecules, 36, 7331 (2003) 4. C. M. Chou, P. D. Hong, Macromolecules, 37, 5596 (2004) 5. M. Manno M. U. Palma phys. Rev. Lett., 79 4286 (1997)

6. H. Fujimatsu and S. Kuroiwa, Colloid Polym. Sci., 265, 938 (1987).

7. H. Fujimatsu, S. Kuroiwa, H. Ihara, T. Takashima, K. Toyaba, and S. Kuroiwa, Colloid Polym.

Sci., 266, 688 (1988).

8. H. Fujimatsu, Y. Ideta, H. Nakamura, H. Usami, and S. Ogasawara Polym. J., 33, 89 (2001) 9. A. Nakai, T. Shiwaku, W. Wang, H. Hasegawa and T. Hashimoto, Polymer, 37 2259 (1996) 10. M. Carpineti and M. Giglio, Phys. Rev. Lett., 70, 3828

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三. 計畫成果自評部份 1. 本年度計畫成果已整理投稿

1. Gelation Behavior and Structural Transition in PLLA/NMP Solutions, Polymer, submitted.

2. Structural Formation and Coloration Mechanism of iso-Poly(1-butene) /o-Xylene Physical Gel, Polymer, submitted.

2. 檢討

本年度計畫已對 Polymer solutions 之 and gelation phase separation 耦合機理利用小 角光散射分析進行深入討論。基本上對於高分子溶液之相分離與凝膠化的研究,本計劃已 得到不錯成果。但由於凝膠結構之分形聚集(Fractal)之學理與實驗較為複雜,數據之解析 與說明至今仍有待努力與檢證。

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