Comparison of thermal techniques for glass transition assignment. II. Commercial polymers

The differential scanning calorimetry glass transition (DSC T_g), measured by ASTM test method E-1356, and the dynamic mechanical analysis glass transition (DMA T_g), measured using a new definition of the DMA T_g, generally agree within ±4°C for a wide variety of commercially available polymers. The DMA T_g is defined as the average of E′ and tan δ peak temperatures measured at a 1 rad/s oscillation frequency. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 191–195, 1997     

The glass transition temperature of nitrated polystyrene/poly(acrylic acid) blends

Low-molecular-weight polystyrene was nitrated to different levels. The nitrated polystyrene was blended with different molecular weights of poly(acrylic acid), PAA. The glass transition temperatures (T_g) for the mixtures were investigated by differential scanning calorimetry. A single T_g was observed for all blends, indicating single-phase blends. In general, it was found that the T_g increases with molecular weight of PAA. The T_g values of the blends showed a positive deviation from the linear average T_g as a result of strong hydrogen bonding between the segments of the component polymers. The observed T_g values were not adequately represented by simple predictive equations or by single-parameter fitting equations. However, two-parameter fitting equations gave a reasonable representation of the data.     

Viscosity of polystyrene near the glass transition

A quantitative explanation is given for the apparent viscosity increase with increasing capillary shear rate for polystyrene at temperatures approaching the glass transition, T_g. Possible shifts in T_g as a function of the parameters shear rate, frequency, and pressure are interrelated to viscosity changes. Experimentally, the Instron capillary rheometer and the Weissenberg rheogoniometer provided a means for uncoupling the variables for individual consideration. Calculated and experimental data for the apparent viscosity as a function of the given parameters are presented and discussed. The explanation of the apparent viscosity increase in capillary flow can be quantitatively explained through the pressure dependence of T_g. Brief mention is made of the pressure effects on the Bagley and Rabinowitsch corrections normally made in capillary measurements.     

聚苯乙烯合成及玻璃化转变特性研究

通过使用乌式黏度计和差示扫描量热仪,分别测定了聚苯乙烯的特性黏度[η]和玻璃化转变温度(Tg),并通过实验数据的回归,得到了其数均相对分子质量(Mn)与Tg的关系。为聚苯乙烯的简易制备和Tg测试提供了一种实用方法。     

The glass transition behaviour and thermodynamic properties of amorphous polystyrene

The effect of molecular weight (MW) and thermal history on the heat capacity (c_p) of amorphous polystyrene over the temperature range from 200–440 K has been measured using differential scanning calorimetry (d.s.c.) General equations are given that show the influence of MW on c_p in both the glassy and liquid states. An enthalpic definition of the glass temperature reflects changes in the glass structure, due to thermal history, that are not found using conventional d.s.c. constructions. Thermodynamic and statistical mechanical theories give reasonable approximations to the observed T_g-MW curve but small systematic deviations are found. The effect of thermal history on T_g can be reproduced using a MW-dependent activation enthalpy.     

Effects of molecular weight and chain ends on glass transition of polystyrene

The main glass transition temperatures, T_g of narrow distribution anionic polystyrene samples have been measured by differential thermal analysis (d.t.a.). The d.t.a. data were verified by differential scanning calorimetry. Effects of heating rates were eliminated by extrapolating observed T_g values to a standard heating mode of 1°C/min In a T_g vs. log (heating rate) relation. The variations of T_g peak height and temperature with heating rate and polymer molecular weight (M) agree with conclusions from other reports of d.t.a. work. The T_g values corrected for heating rate effects agree well with figures reported in studies with d.t.a. and other techniques for locating vitrification temperatures.The volumes attributable to end groups differ significantly between anionic and thermally initiated polystyrenes at temperatures above T_g. A comparison of T_g values of both polystyrene types provides a direct test of the iso-free volume model of the vitrification process. A common T_g vs. M relation was found for anionic and thermally initiated polystyrenes, regardless of chain end nature. This comparison is not consistent with the concept that vitrification is governed by free volume defined as the difference between specific volume of the polymer and a reference volume. Our results are compatible with several other mechanisms which have been suggested for the glass transition.     

Morphology and glass transition of thin polylactic acid films

The aim of this study is to highlight the effect of the morphology of polylactic acid (PLA) thin films on the glass transition in this confined geometry. For that purpose, poly(L ‐lactic acid) and poly(D ‐lactic acid) polymer films were prepared on different surfaces such as mica and silicon surfaces by using two different solvents. The films exhibit different morphologies starting from individual macromolecular chains at very low concentration to homogeneous films at higher concentrations. Globular to elongated rodlike structures of the PLA macromolecules were observed according to the polarity of the surface and the solvent. The T_g of PLA enantiomeric film, determined by adhesion force using atomic force microscopy (AFM) as a function of temperature, was confirmed by ellipsometric measurements. Surprisingly, this value is independent of the morphology of thin enantiomeric film. A balance between the attractive surface effects and a decrease of the crystallinity of PLA on the mobility in the amorphous region will be discussed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Comparison of glass transition measurements between carrimed rheometer and differential scanning calorimetry using methylacrylate-vinylidene chloride copolymer

The potential of the Carrimed CSL 500 rheometer for the thermal analysis of methylacrylate-vinylidene chloride (MA-VDC) random copolymer was evaluated by comparing the glass transition temperature (T_g) of the copolymers with the value obtained from the differential scanning calorimeter. The major relaxation phenomenon in amorphous polymers, namely the glass transition temperature, could be identified clearly using the Carrimed rheometer. Samples of varius contents of methylacrylate in methylacrylate-vinylidene chloride copolymer were prepared as solvent cast films. Small amplitude oscillatory measurements showed as a function of temperature, that methylacrylate-vinylidene chloride copolymer between 12% and 70% methylacrylate (MA) content showed glass transition temperatures between 48°C and 63°C. For the methylacrylate content ranging from 4% to 70%, the saran copolymers had glass transition temperatures between 12°C and 56°C. Both techniques showed the T_g of the methylacrylate-vinylidene chloride copolymer first increased and then decreased with increasing methylacrylate content. Both methods show a peak in the T_g vs. percent methylacrylate content of the copolymer at around 50% methylacrylate content. Small angle amplitude measurements also showed that it is very sensitive to the frequency. The T_g obtained using the Carrimed CSL 500 rheometer is very reproducible and is comparable with that obtained using the standard DSC.     

Thermal conductivity and thermal expansion behavior of glass fiber-reinforced rigid polyurethane foam

The thermal conductivities and the thermal expansion curves of glass fiber-reinforced rigid polyurethane foams with various fiber lengths, various fiber volume fractions and various matrix densities were determined experimentally. Additionally the thermal expansion coefficients of these materials at room temperature were examined in terms of the interaction between fiber and matrix. The thermal expansion properties were analyzed successfully with the analogous treatment which is applied to the mechanical tensile behavior.     

Influence of silica reinforcement upon the glass transition behavior of acrylic polymers

The glass transition temperatures (T_g) of poly(acrylic acid), poly(methyl acrylate), and poly(ethyl acrylate) filled with submicron particulate silicas and silicates have been measured by dynamic mechanical spectroscopy and differential scanning calorimetry. The peak temperatures of the damping factor (tan δ) and the dynamic shear loss modulus (G″) were shifted by an amount which depended upon the quantity and type of filler added to each polymer. The temperatures corresponding to the step discontinuity of specific heat also shifted, but to a lesser extent than those measured mechanically. The degree of T_g shift per unit of volumetric filler addition increased with polymer pendant group polarity for both measurement methods. Utilizing a T_g-crosslinking analogue, a model was developed that related positive T_g shifts to polymeric segmental adsorption onto filler surfaces. This model also incorporated negative contributions to the T_g shift from energy storage mechanisms arising from particle–particle interactions, as well as corrections due to effective surface area available for polymer adsorption.     

High strain rate mechanical properties of a cross-linked epoxy across the glass transition

Molecular dynamics simulations were used to study the high strain rate mechanical properties of a cross-linked epoxy system comprised of diglycidyl ether of bisphenol A (DGEBA) that is cross-linked by a poly(oxypropylene) diamine with three propylene oxide moieties per diamine. Atomistic network structures were characterized using volume-temperature behavior and their response to mechanical deformation. The Young's modulus was determined as a function of temperature across strain rates spanning three decades in magnitude, and collapsed onto a single “master curve” using the time–temperature superposition principle (TTSP). The master curve obtained from molecular dynamics simulation data shows good agreement with a similar master curve of the reduced storage modulus as a function of frequency, which was obtained using experiments. At higher strain rates, the simulation master curve deviated from the experimental master curve. This deviation could be attributed to the lack of occurrence of sub-T_g motions on the time scale of simulations due to the use of higher strain rates in simulations compared to experiments. Our work demonstrates the utility of TTSP in connecting the thermo-mechanical behavior of polymers at high strain rates and high temperatures to experiments performed at much different conditions. To the best of our knowledge, the use of the time–temperature superposition to compare mechanical properties determined from molecular simulation and experiments is the first reported effort of its kind.     

PVC用UV固化聚氨酯丙烯酸酯胶粘剂的研究

采用聚氨酯丙烯酸酯预聚体,添加光引发剂、活性稀释剂、增塑剂及偶联剂等制成PVC用UV固化胶粘剂。结果表明,当活性稀释剂质量分数为20％-25％,且mHDDA：mTGPDA：mPO3TMPTA=2：2：1、硅烷偶联剂质量分数为3％-5％、光引发剂用量为3％～5％,且m1173：m184=1．0：1．5时,用自制表面处理剂对PVC进行处理,胶粘剂粘接的剪切强度为15MPa．综合性能较好。适合于软质PVC的粘接。     

Kinetics of acrylic films photopolymerization through analysis of the thermal curve

Kinetic curves of polymerization were obtained through a photocalorimetric device that allows thermal measurements on flat films of controlled thickness of oligomers based on bis‐phenol‐A‐diglycidyl‐ether diacrylate. The temperature of the film was monitored during the ultraviolet irradiation under controlled conditions and the resulting thermal curve was processed by a suitable thermokinetic model based on the balance between the internal energy generation and the heat flow at the surfaces. The kinetic behavior of the acrylic group polymerization agrees with the kinetics followed by Fourier transform infrared analysis and fulfills the kinetic behavior expected for a photopolymerization process under steady state conditions. The proposed technique should be a suitable method to study in real time the reactivity of ultraviolet crosslinkable films in different condition of process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 458–463, 2000     

Study on glass transition and physical aging of polystyrene nanowires by differential scanning calorimetry

In the present study, polystyrene (PS) nanowires in aqueous dispersion with a diameter ranging from 120 to 300 nm are prepared via electrospun and dispersed in water. Relatively, PS nanowires confined in anodic aluminum oxide (AAO) template are prepared through drawing PS flow into the tube of AAO template. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) are used to characterize the size of the resultant nanowires and ensure the successful preparation of nanowires. Moreover, the glass transition and physical aging behaviors of the PS nanowires in aqueous dispersion and confined in AAO template are investigated by differential scanning calorimetry (DSC). The PS nanowires in aqueous dispersion and confined in AAO template exhibit different glass transition and physical aging behaviors.     

Study on glass transition temperature and mechanical properties of cadmium sulfide/polystyrene nanocomposites

The cadmium sulfide/polystyrene (CdS/PS) nanocomposites with concentration (0, 2, 4, 6, and 8) wt% of CdS nanoparticles were prepared by solution casting method and characterized through fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) measurements. The particle size of nanoparticles is found to be around 15 nm. Glass transition and mechanical behavior of CdS/PS nanocomposites were investigated using dynamic mechanical analyzer (DMA). The mechanical properties such as Young's modulus and tensile strength were determined at room, as well as at elevated temperatures through their stress–strain curves. The result shows that glass transition temperature (T_g) is shifted toward the higher temperature after the addition of CdS nanoparticles. The mechanical properties increased at low wt% loading of CdS nanoparticles and decreased for higher wt% loading of CdS nanoparticles. It was also found that mechanical properties decline with increase in the temperature. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Absorption and diffusion measurements of water in acrylic paint films by single-sided NMR

Artists’ acrylic paint surfaces can accumulate surface grime. Aqueous surface cleaning of these paints can be particularly problematic because they swell upon exposure to water and because of the presence of water extractable additives. Research to determine the parameters of water penetration and swelling as well as the role of the water soluble additives in these phenomena is important in order to understand the leaching mechanism and possibly describe the best conditions for removal of water soluble grime. In this work, the penetration and molecular dynamics of water with different salt concentrations in films of artists’ waterborne acrylic paint were studied for the first time in situ using noninvasive single-sided nuclear magnetic resonance (NMR). This novel approach demonstrated that the absorption of water in the acrylic film is independent of the salt concentration, whereas the absorption of water in water-washed acrylic paints, after the water soluble components have been removed, is not only 15% less than unwashed paint films but it also decreases with increasing salt concentration. In addition, the self-diffusion coefficients of water within the acrylic polymeric network were determined and analyzed. The diffusion coefficients of water depend only slightly on the salt concentration and are only marginally influenced by the amount of incorporated surfactants. Thus the diffusion rate of water within the acrylic polymer film appears to be almost independent of the salt concentration or the surfactant content.     

Studies on glass transition temperature of chitosan with four techniques

Studies on the glass transition temperature (T_g) of chitosan are difficult to pursue because of the difficulty in sample preparation and the hydroscopicity of samples. There are a few works concerning this principal relaxation of chitosan. Among them, several quite different values (150°C, 161°C, and 203°C) have been reported. In this paper, the T_g of chitosan (140 ～ 150°C) was determined by means of four techniques, namely, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), thermally simulated current spectroscopy (TSC), and dilatometry (DIL). DSC measurement has been assumed not to be sensitive enough to detect the relaxation temperature of polysaccharides. We propose a new method to improve the sensitivity of the DSC measurement. After a physical aging treatment of samples, the transition in DSC traces became much more distinct because of the enthalpy relaxation. This technique was also used to distinguish the T_g from other relaxations. The T_g of chitosan with different degree of deacetylation (D.D.) was examined by DSC. No influence of D.D. on T_g was found. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1553–1558, 2004