4.1.1. Volume shrinkage for St/VER/gp-CSR systems
The effects of submicron and nano-scale core-shell rubber (CSR) ranging from 30 nm to 240 nm in size as low-profile additives (LPA) on the volume shrinkage characteristics for low-shrink vinyl ester resins (VER) during the cure at 110oC were investigated. These CSRs were synthesized by two-stage emulsion polymerizations, where the soft core was made from rubbery poly(n-butyl acrylate) (PBA), while the hard shell was made from methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDMA) as a crosslinking agent, and varied amounts of glycidyl methacrylate (GMA) as a comonomer.
By employing submicron and nano-scale CSR ranging from 30 nm to 240 nm in size and with the shell composition modified by 0 to 10 mole % of GMA, a reduction of cyclization reaction for VER during the cure could be facilitated. The microgel structure during the cure would thus be less compact due to the segregating effects of CSR on microgel structures, and, in turn, be favorable for the decrease of intrinsic polymerization shrinkage after the cure. Moreover, the relief of the polymerization shrinkage force caused by the rubbery core of the CSR may lead to the much less compact microgel structures, and even an expansion in volume change after the cure could be observed.
The best volume shrinkage control has been achieved for the 10 wt% G2-60 CSR system with a fractional volume shrinkage of 0.2 %.
4.1.2 Volume shrinkage for Epoxy/DDM/gp-CSR systems
The volume shrinkage of the neat Epoxy/DDM was about 2.4%, while adding 5 or 10 wt % different CSRs, such as G-30, G-60, and G-240, can reduce the volume shrinkage to the range of -3.8% (volume expansion) to 0.85%. In general, a higher concentration of CSR would lead to a lower volume shrinkage. Also, adding 10wt% of G0-30 or 10 wt% of G0-60 could result in a volume expansion as high as 3.8%, which is due to very good segregrating effects of CSR on microgel structures. The white spots observed in the TEM micrographs are due to the microvoids generated during the cure, which could verify the relief of the polymerization shrinkage force caused by the rubbery core of the CSR.
4.1.3 Mechanical properties for St/VER/gp-CSR and epoxy/DDM/gp-CSR systems
In general, the addition of nano-scale gp-CSR (30 nm and 60 nm) and the submicron (240 nm)
gp-CSR in this work would not be able to enhance the impact strength, Young‟s modulus, and tensile strength for the St/VER and Epoxy/DDM systems although most of the gp-CSR used in this work may effectively reduce the volume shrinkage during the cure.
The nano-scale CSR would need to modified in such a way that the shell material of the CSR exhibits a rubbery state to be an effective toughener for St/VER and Epoxy/DDM systems.
4.2 Second Year
4.2.1 Synthesis of nano-scale living core-shell rubber
The effects of nano-scale living core-shell rubber (CSR) ranging from 15 nm to 100 nm in size as low-profile additives (LPA) on the volume shrinkage characteristics for low-shrink vinyl ester resins (VER) and epoxy/DDM (4,4‟-diaminodiphenylmethane) resins were investigated.
These CSRs designated as BA/St-SA, with poly(butyl acrylate) as the core and
poly(styrene-co-sodium acrylate) as the shell, were synthesized by emulsifier-free reversible addition-fragmentation chain transfer (RAFT) seeded emulsion polymerizations of butyl acrylate from poly(styrene-co-sodium acrylate) dispersions, which were made by a first bulk
copolymerization of styrene (St) and acrylic acid (AA), with dibenzyltrithiocarbonate (DBTTC) as a RAFT agent, followed by the addition of a sodium hydroxide solution inducing a spontaneous phase inversion.
In the synthesis of final dispersion of the nano-scale SAA latex, the size of the SAA latex can be controlled by adjusting the pH value (ca. 12) prior to the reaction, while that of the BA-SAA latex, which is made by the further chain extension by n-butyl acrylate (BA) to obtain the BA-SAA type of CSR, can also be controlled by pH (ca. 8-13) prior to the synthesis of BA-SAA latex. 15 nm, 60 nm and 100 nm of nano-scale s-CSR (i.e BA-SAA latex) have been synthesized by RAFT emulsion polymerizations in this work.
4.2.2. Volume shrinkage for St/VER/s-CSR and Epoxy/DDM/s-CSR systems
For the St/VER/s-CSR systems, adding 5% or 10% of BA/St-SA type of living CSR with 15 nm in size (i.e. E7-15nm) is unfavorable for the volume shrinkage control since not enough or too much phase separation during the cure would result. This is also true for the Epoxy/DDM/5%
E7-15nm system, which exhibited a noticeable phase separation during the cure. Some phase separation during the cure would be indispensable to have a good volume shrinkage control.
For the Epoxy/DDM/s-CSR system, by employing 10 wt% E7-15nm, a pertinent compatibility during the cure would result, and a reduction of cyclization reaction for Epoxy/DDM during the cure could be greatly facilitated. The microgel structure during the cure would thus be less compact due to the segregating effects of CSR on microgel structures, and, in turn, be favorable for the decrease of intrinsic polymerization shrinkage after the cure. Moreover, the relief of the polymerization shrinkage force caused by the rubbery core of the CSR may lead to the much less compact microgel structures, and can result in a good volume shrinkage control (-ΔV/V0 = 0.38 %).
4.3 Third Year
4.3.1 Effects of micron and nano-scale inorganic/organic core-shell particle on the volume shrinkage in the cure of unsaturated polyester and vinyl ester resins
The effects of micron and nano-scale inorganic/organic core-shell particle (CSP) as low-profile additives (LPA) on the volume shrinkage characteristics for low-shrink unsaturated polyester (UP) and vinyl ester resins (VER) during the cure at 110oC were investigated. These CSP designated as Si-polymer, which contained silica particle as the core and organic polymer as the shell, were synthesized by the Z supported reversible addition-fragmentation chain transfer (RAFT) graft polymerization using silica-supported 3-(benzylsulfanylthiocarbonylsulfanyl) propionic acid (Si-BSPA) as the chain transfer agent (CTA).
By employing nano-CSP types of LPA (i.e. 5-15 nm size of Si-poly(methyl acrylate)), as can be synthesized by Z-supported RAFT polymerizations, the interaction between UP and nano-CSP
during the cure could be facilitated, leading to a reduction of cyclization reaction for UP resin.
The compact microgel structure of the styrene-crosslinked polyester network can then be alleviated, leading to a considerable decrease in the intrinsic polymerization shrinkage and, in turn, a better volume shrinkage control.
At 1 wt% of the nano-CSP (i.e. 5-15 nm size of Si-poly(methyl acrylate)) , as the molecular polarity difference between the resin matrix and the shell polymer of CSP increased, the fractional volume shrinkage was generally decreased, followed by an increase, and reached a minimum of 5.9% at a polarity difference of 0.0067 debye/cm3/2 for the MA-PG type of UP system.
4.3.2 Synthesis of colloidal silica nanoparticles via hydrolysis of elemental silicon and synthesis of
Si-PMA and Si-PBA-b-PMA by RAFT solution polymerizations
Several uniform-size nano-scale silica particles ranging from 15 to 60 nm in diameter have been successfully synthesized using a two-stage hydrolysis of silicon powder in aqueous medium, which consists of nucleation and regrowth. The sizes of the nanaparticles can be controlled by using a regrowth procedure.
3-(benzylsulfanylthiocarbonylsulfanyl) propionic acid (BSPA), which is a chain transfer agent for the RAFT polymerization in this work, has also been synthesized and purified according to literature methods.
The synthesis of polymer-grafted silica particles, namely, inorganic/organic core-shell particle (CSP) employed as low-profile additives (LPA) for low-shrink unsaturated polyester (UP), vinyl ester resins (VER), and epoxy (EP), have been synthesized by Z-supported RAFT polymerizations in this work. The following key stepsare involved in the synthesis of Si-polymer:
Synthesis of BSPA-grafted silica particles (Si-BSPA), where silica was reacted with 4-(chloromethyl)phenyltrimethoxysilane to produce benzyl chloride functionalized silica (Si-Cl) first, followed by reacting with BSPA to make the Si-BSPA , (ii) RFAT solution polymerization of methyl acrylate mediated by Si-BSPA at 60oC for 18-21 hr, with or without adding free BSPA in the reacting mixtures, and (iii) aminolysis to cleave the grafted polymer chains on the silica gel for the characterization of molecular weight and molecular weight distribution by GPC, and (iv) structure characterization of BSPA, Si-Cl, Si-BSPA, Si-poly(methyl acrylate) (i.e. Si-PMA) by FTIR, H1 NMR, C13 NMR, GPC, TGA, EA, and DSC.
The synthesis of 15 nm Si-PMA (i.e. Si-PMA15nm) and 30 nm Si-PMA (i.e. Si-PMA30nm) using the self-synthesized of silica nanoparticle in our laboratory has been completed, with the number-average molecular weight (Mn) of PMA ranging from 800 to 1400 g/mol. Via chain extension polymerization of butyl acrylate (BA), diblock copolymer-silica hybrids, namely, 15 nm Si-PBA-b-PMA (i.e. Si-PBA-b-PMA15) and 30 nm Si-PBA-b-PMA (i.e. Si-PBA-b-PMA30), have also been successfully synthesized, with Mn ranging from 2500 to 2800 g/mol..
4.3.3 Mechanical properties for St/VER/Si-PMA and St/VER/Si-PBA-b-PMA cured systems
For the St/VER/Si-PMA-15nm cured systems, adding Si-PMA15 would decrease the impact strength and tensile strength. Also, the higher the concentration of Si-PMA15, the lower the impact strength and the tensile strength would be. In contrast, as the concentration of Si-PMA15 was increased from 0 to 5 wt%, the Young „s modulus would increase, followed by a decrease, and reach a maximum value at a concentration of 2.5% .
For an effective toughener of nano-scale core-shell rubber (CSR) or core-shell particle (CSP), the Young‟s modulus for its shell polymer would need to be adjusted so that it exhibits a rubbery state at the use temperature. Otherwise, a good toughening for the resin matrix would not be able to be achieved.
Si-PBA-b-PMA15 was synthesized by chain extension of butyl acrylate monomer on the living Si-PMA core-shell particles, with a view to introducing a polymeric segment of poly(butyl
acrylate) (PBA) as the interlayer of the CSP and improving the toughening properties of the St/VER/Si-PBA-b-PMA systems. Indeed, the impact strength for 1% Si-PBA-b-PMA15 system was higher than that of 1% Si-PMA15 system by about 30%, but at the expense of Young‟s modulus and tensile strength by about 10% when compared with that of 1% Si-PMA15 system.
Apparently, the Si-PBA-b-PMA15 type of nano-scale CSP shows promising in the toughening of St/VER resin system. More effort should be devoted in the future to the synthesis of polymer-grafted nano-scale silica particle by RAFT polymerizations in the improvement of toughening properties for both St/VER and Epoxy/DDM systems.
The design parameters include the choice of polymer segments in the CSP shell and interlayer with different molecular weights, and the introduction of reactive functional groups in the polymer shell to enhance the interfacial adhesion between the resin matrix and the CSP, and so on.
Since the density of the CSP, which is a polymer-grafted silica particle in our case, is usually much larger than that of the resin matrix, the design of the resin, such as VER and Epoxy, with a higher viscosity would be indispensable to reduce the sedimentation rate of the CSP during the preparation of the St/VER/CSP or Epoxy/DDM/CSP systems. Thickening process for unsaturated polyester resin (UP) may even need to be adopted for the St/VER/CSP and Epoxy/DDM/CSP systems in the sample preparation in order to prevent a too fast phase separation for the CSP prior to gelation during the cure.
4.3.4 Volume shrinkage for St/VER/Si-PMA cured systems
The effects of particle size of CSP (i.e. Si-PMA) and CSP content on the fractional volume shrinkage for St/VER/Si-PMA systems cured at 110oC have been investigated. Adding either 5%
or 10% of Si-PMA with varied particle size (15 nm or 30 nm) could not reduce the volume shrinkage appreciably. Hence, more effort will need to be devoted in the future in the study of effects of nano-scale Si-polymer, such as Si-PMA, on the volume shrinkage of vinyl ester resins.
How to prevent or slow down the phase separation of Si-PMA from the original St/VER/Si-polymer system prior to gelation during the cure and how to effectively segregate the microgel particles by the Si-PMA during the cure of the St/VER/Si-polymer systems would be the key factors in the development of the Si-polymer as an effective low profile additive (LPA) in the cure of vinyl ester resins.
4.3.5 Synthesis of core-shell rubber additives of MA-Gx type by conventional emulsion polymerizations
The nano-scale core-shell rubbers (CSR), with poly(butyl acrylate) (PBA) as the core and poly(methyl acrylate) (PMA) as the shell, were synthesized by two-stage emulsion polymerizations.
The shells of the CSR were also modified by introducing ethylene glycol dimethacrylate (EGDMA) as a crosslinking agent with or without glycidyl methacrylate (GMA) as a comonomer. Six general-purpose CSRs, including two different size of CSR (30 nm and 60 nm in diameter) and three levels of GMA content introduced in the polymer shell (0%, 5%, and 10% by mole), have been synthesized, namely, BA/MA-EGDMA (i.e. MA-G0 type), BA/MA-EGDMA-GMA(5) (i.e.
MA-G1 type), and BA/MA-EGDMA-GMA(10) (i.e. MA-G2 type). These CSRs can be employed as toughener for thermoset resins, such as epoxy/DDM and styrene/VER resins.
4.3.6 Mechanical properties for Epoxy/DDM/gp-CSR(MA-Gx-30) cured systems
The 10% MA-G0-30 system can lead to an increase of impact strength by 25% and an increase of fracture energy by 80%, only at the expense of a decrease in Young‟s modulus and tensile strength by 15-20%. For the nano-scale core-shell rubber with diameter ranging from 30 nm to 100 nm, adjusting the Tg for the shell of CSR in such a way that the polymer shell is in the rubbery state at the use temperature would be favorable for in the enhancement of impact strength
and fracture energy for the CSR-toughened epoxy system. In the future, more effort will be devoted to the toughening of epoxy and vinyl ester resin systems by nano-scale core-shell rubber (CSR).
4.3.7 Curing behavior and properties for neat St/VER and Epoxy/DDM systems 4.3.7.1 Cured sample morphology for neat St/VER and Epoxy/DDM systems by SEM
The dependency of the fractured surface of SEM micrographs for neat St/VER cured systems on varied molar ratio (MR) of styrene to vinyl ester C=C bonds, with MR = 1/1, 1.5/1, 2/1, and 2.5/1, has been studied. At MR = 1/1, the microgel structure can be clearly seen. As the MR was increased to 1.5/1, the microgel structure remained visible, but the average size of the microgel structure was reduced. Increasing the MR to 2/1 would lead to a flake-like microstructure, and the microgel structure cannot be clearly identified. Further increasing the MR to 2.5/1 could result in a swollen microgel structure. Apparently, the change of cured sample morphogy is a result of the change in the concentration of styrene monomer which may exert a swelling effect on the microgel structure. When the swelling effect is inadequate, such as the case of MR = 1/1 and MR = 1.5/1, individual compact microgel structure can then be identified. At MR = 2/1, the swelling effect on the microstructure would be pertinent so that a flake-like microstructure was observed. When the swelling effect on the microgel structure is more than enough, such as the case of MR = 2.5/1, swollen microgel structure can be observed.
The dependency of the fractured surface of SEM micrographs for neat Epoxy/DDM cured systems on varied equivalent ratio (ER) of epoxy group to active hydrogen in DDM, with ER = 0.5/1, 0.75/1, 1/1, 1.25/1, and 1.5/1, has been investigated. At ER = 0.5/1, a compact microgel structure was observed. As the ER was increased to 0.75/1, a flake-like microsture was seen along with some microgel structure scattered around. Increasing the ER to 1.25/1 would lead to a swollen microstructure, and the local microgel structure cannot be connected together to span the whole sample space. Further increasing the ER to 1.5/1 could result in a swollen microgel structure somewhat resembling that of ER at 1.25/1, but with a much lower crosslinking density for the interwoven three-dimensional network. Apparently, the change of cured sample morphogy is a result of the change in the concentration of epoxy resin (with a functionality of 2) which may exert a swelling effect on the microgel structure. When the swelling effect is inadequate, such as the case of ER = 0.5/1, individual compact microgel structure can then be identified. At ER = 0.75/1 and 1/1, the swelling effect on the microstructure would be pertinent so that a flake-like microstructure was observed. When the swelling effect on the microgel structure is more than enough, such as the case of ER = 1.25/1 and 1.5/1, swollen microgel structure or even swollen interwoven microgel structure can be observed.
4.3.7.2 DSC cure kinetics for neat St/VER and Epoxy/DDM systems
According to the isothermal DSC reaction profiles at 110oC for neat St/VER binary systems at varied molar ratio (MR) of styrene to vinyl ester C=C bonds, with MR = 1/1, 1.5/1, 2/1, and 2.5/1, as the MR was increased from MR = 1/1 to 2.5/1, the maximum DSC reaction rate was increased, followed by a decrease, and reached a maximum at MR = 2/1. The DSC final conversion of total C=C bonds as a function of MR exhibits the same trend as that for the DSC maximum reaction rate.
Based on the isothermal DSC reaction profiles at 100oC for neat Epoxy/DDM binary systems at varied equivalent ratio (ER) of epoxy group to active hydrogen in DDM, with ER = 0.5/1, 0.75/1, 1/1, 1.25/1, and 1.5/1, the maximum DSC reaction rate was decreased monotonically. The epoxy was served as both a reagent and a solvent. As the ER (or epoxy concentration) was increased, the concentration of epoxy and DDM was decreased, leading to a decrease in the cure reaction rate.
Moreover, when ER = 0.5/1, the active hydrogen in DDM would be 100% in excess. Hence, only the active hydrogens from the primary amine would be sufficient to react with epoxy groups, and no active hydrogens from the secondary amine would react with epoxy groups. As ER was increased
from 0.5/1 to 1/1, the relative percentage of active hydrogens from the secondary amine participating the cure reaction with epoxy groups was increased. Since the reaction rate of primary amine is greater than that of secondary amine during the cure reaction between epoxy and DDM, the DSC maximum reaction rate would be reduced with increasing ER. As ER was larger than the stoichiometric ratio of 1/1, the DSC maximum reaction rate would decrease with increasing ER due to the dilution effect of epoxy as a solvent as mentioned earlier.
At ER = 1/1, the DSC final conversion of limiting functional group can reach about 85%.
4.3.7.3 Glass transition temperature for neat St/VER and Epoxy/DDM systems as measured by DMA
For St/VER systems, the glass transition temperature for the overall styrene-crosslinked vinyl ester three-dimensional network, as identified by the maximum point of the tan curve and ranging from 144oC to 172oC, was decreased as MR was increased form MR = 1/1 to 2.5/1 At MR = 2/1, the Tg for the styrene-crosslinked vinyl ester network in this work was found to be 151.2oC.
For Epoxy/DDM systems, as the ER was increased from 0.5/1 to 2.5/1, the glass transition temperature for the overall DDM-crosslinked epoxy three-dimensional network, as identified by the maximum point of the tan curve and ranging from 94oC to 169oC, was increased, followed by a decrease, and reached a maximum at ER = 1/1. At ER = 1/1, the Tg for the DDM-crosslinked epoxy network in this work was found to be 168.8oC.
4.3.7.4 Mechanical properties for neat St/VER and Epoxy/DDM cured systems
For St/VER systems, as the MR was increased from MR = 1/1 to 2.5/1, the impact strength was increased, whereas the Young‟s modulus and tensile strength was generally decreased.
For Epoxy/DDM systems, at an ER range of 0.5/1 to 1/1, as ER was increased, the impact strength was increased, followed by a decrease, and reached a maximum at ER = 0.75/1. Again, at an ER range of 1/1 to 1.5/1, as ER was increased, the impact strength was increased, followed by a decrease, and reached a maximum at ER = 1.25/1. Among the five ER‟s studied, the maximum impact strength occurred at ER = 0.75/1.
For Epoxy/DDM systems, at an ER range of 1/1 to 1.5/1, as ER was increased, the fracture energy was increased, followed by a decrease, and reached a maximum at ER = 1.25/1, which revealed the same tendency as that of impact strength. Among the five ER‟s studied, the maximum fracture toughness occurred at ER = 0.5/1 for the Epoxy/DDM cured systems, which is in contrast to the maximum impact strength at ER = 0.75/1.
For neat Epoxy/DDM cured systems, the trend of fracture energy change as a function of ER remained the same as that of fracture toughness
For neat Epoxy/DDM systems, in general, as the ER was increased from ER = 0.5/1 to 1.5/1, both Young‟s modulus and tensile strength were decreased, followed by an increase, and reached a minimum at ER = 1/1. Among the five ER‟s studied, both the maximum Young‟s modulus and the maximum tensile strength occurred at ER = 0.5/1.