Transitions from solid to liquid in isotactic polystyrene studied by thermal analysis and X-ray scattering

A three-phase model, comprising mobile amorphous fraction (MAF), rigid amorphous fraction (RAF) and crystalline fraction (C), has been applied to interpret the thermal transitions and structure of cold-crystallized isotactic polystyrene (iPS) from below the glass transition temperature, T_g, to above the melting point. Quenched amorphous iPS films were isothermally crystallized at different temperatures for 12 h. The fraction of crystalline phase, ?_c, was measured by differential scanning calorimetry (DSC), wide angle X-ray scattering and Fourier Transform infrared spectroscopy. The fraction of the mobile amorphous phase, ?_MAF, was obtained from the heat capacity increment at the glass transition temperature. In the three-phase model, the fraction of the rigid amorphous phase, ?_RAF, was found from 1−?_MAF−?_c. Specific heat capacity measurements by standard DSC confirm that the experimental baseline heat capacity conforms to a three-phase model for temperatures ranging from below T_g, up to the relaxation of RAF. The relaxation of RAF appears as a sigmoidal change in heat capacity accompanied by excess enthalpy, in which solid-like RAF is converted to an identical amount of liquid-like MAF.At temperatures above the relaxation of RAF, either one or two major crystal melting endotherms are observed in standard DSC, dependent upon crystallization temperature. However, using quasi-isothermal temperature modulated DSC, we always observed two reversing melting endotherms. The effects of annealing on iPS structure during the quasi-isothermal measurement were assessed using small angle X-ray scattering (SAXS). Combining the DSC and SAXS results, a model for the melting of iPS lamellae at low heating rates is presented.     

The measurement of the crystallinity of polymers by DSC

The procedures adopted and the inherent assumptions made in the measurement of crystallinity of polymers by differential scanning calorimetry (DSC) are reviewed. The inherent problem in all DSC measurements is concurrent recrystallisation and melting of the polymer sample on heating to the melting point and the variation of the enthalpies of crystallisation and melting, heat capacities and degree of crystallinity with temperature. A First Law procedure is suggested which involves heating the sample between two set temperatures, T₁ and T₂. T₁ is selected by the requirement that the degree of crystallinity of the sample should not change either with temperature or time, and be representative of the sample during its use. T₁ is taken to be ambient or just above the glass transition temperature. T₂ is taken to be just above the observed last trace of crystallinity. Integrating the observed specific heat difference between the sample and the completely amorphous material during these two temperature ranges determined the residual enthalpy of fusion at T₁. Problems are noted in the use of this procedure in that the specific heat of the liquid should not be arbitrarily chosen since this leads to systematic errors in the heat of crystallisation.The degrees of crystallinity of metallocene polyethylene (m-PE) and polyethylene terephthalate (PET) measured by this procedure have been compared with values measured by density, determined at room temperature.     

Crystallization characteristics of weakly branched poly(ethylene terephthalate)

A series of branched poly(ethylene terephthalate) (BPET) samples were prepared from melt polycondensation by incorporation of various amount (0.4-1.2 mol%) of glycerol as a branching agent. These polymers were characterized by means of H¹ NMR, intrinsic viscosity. The general crystalline and melting behavior was investigated via DSC. It was found that the crystalline temperature T_cc from the melt shifted to high temperature and the T_hc from the glass got low for BPETs while the melting temperatures of BPETs kept almost unchanged. The kinetics of isothermal crystallization was studied by means of DSC and POM. It was found that the present branching accelerated the entire process of crystallization of BPETs, although prolonged the induced time. In addition, branching reduced nucleation sites; hence the number of nucleates for BPET got smaller. Therefore, more perfect geometric growth of crystallization and greater radius of spherulites could develop in BPET due to less truncation of spherulites.     

Thermally stimulated recovery of plastic strain in crosslinked and uncrosslinked epoxy/amine systems

Effects of crosslinks on plastic strain recovery in epoxy glass was studied by means of thermally stimulated strain recovery (TSSR) and differential scanning calorimetry (DSC) techniques. Two types of samples were prepared from bisphenol-A type epoxide monomer: one had a crosslinked structure cured by 4,4′-diaminodiphenylmethane and the other had a linear uncrosslinked structure polymerized by aniline. Both specimens were compressed first in the rubbery state and subsequently compressed further in the glassy state. After compressed in the glassy state, the specimens were subjected to the TSSR and the DSC measurements. The TSSR results indicated that the amount of plastic strain recovering at temperatures below the glass transition temperature T_g was larger for the crosslinked sample than for the linear sample. On the other hand, the DSC results indicated that the amount of exothermic heat flow at temperatures below T_g was less for the crosslinked sample than for the linear sample. These results presumably indicate that, for the crosslinked epoxy glass deformed in the glassy state, glass-like strain recovers quite cooperatively with rubber-like strain at temperatures below T_g, in contrast to independent recovery of these strains in the linear epoxy glass.     

Frictional surface temperature determination of high-temperature-resistant semicrystalline polymers by using their double melting features

Most semicrystalline polymers exhibit multiple melting peaks in the course of normal differential scanning calorimetry (DSC) measurements. When their amorphous versions are annealed above the glass transition temperatures, the lower endothermic temperatures (T_m1) appearing on the subsequent DSC heating traces are highly dependent on the annealing temperature (T_a). In consideration of the fact that temperature is the critical environmental factor controlling polymer crystallization, thermal history experienced by the material during annealing in the DSC cell is basically equivalent to that under frictional heating, and the surface temperature prevailing under sliding wear can be estimated from DSC scans taken on the worn surface. In this case, the lower melting peak temperature observed (which can be correlated with the annealing temperature) serves as an indicator for the flash temperature. In addition, this thermoanalytical method can also provide information about microstructural changes due to wearing. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 589–593, 1997     

Characterization, crystallization kinetics and melting behavior of poly(ethylene succinate) copolyester containing 5 mol% trimethylene succinate

Copolyester was synthesized and characterized as having 94.4 mol% ethylene succinate units and 5.6 mol% trimethylene succinate units in a random sequence as revealed by NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics of this copolyester in the temperature range (T_c) from 30 to 80 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC) by varying the T_c, the heating rate and the crystallization time. DSC and TMDSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are primarily due to the melting of lamellar crystals with different stabilities. A small exothermic curve between the main melting peaks gives a direct evidence of recrystallization. As the T_c increases, the contribution of recrystallization gradually decreases and finally disappears. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 108.3 °C. The kinetic analysis of the spherulitic growth rates indicated that a regime II → III transition occurred at ∼65 °C.     

Nylon 6/66/11 Copolymer Used for Hot-Melt Adhesives: Synthesis and Properties

A new copolyamide, nylon 6/66/11, used for hot-melt adhesives, was prepared by hydrolytic polymerization and melt polycondensation. Intrinsic viscosity, melting point, glass transition temperature, cold crystallization, thermogravimetry (TG) and adhesion strength of the resultant hot-melt adhesives were investigated. DSC thermograms of the copolyamide showed that both the melting point and glass transition (T _g) temperatures decreased as the molar fraction of aminoundecanoic acid increased. The T _g practically did not change as the mole fraction of aminoundecanoic acid increased from 60% to 80%, but cold crystallinity of the copolyamide decreased. The thermal gravimetric analysis indicated that the resultant copolyamide had a high thermal stability. The copolyamide had the best combination of properties when the molar fraction of aminoundecanoic acid was about 65%.     

Exploration of the transition temperatures and crystal structure of highly crystalline poly(1,3-cyclohexadiene): An experimental and computational investigation

Experimental determination of transition temperatures for highly crystalline polymers such as poly-1,3-cyclohexadiene (PCHD) can be difficult due to reduced solubility and thermalization processes which occur during data acquisition. In order to facilitate further understanding of these processes for PCHD, density functional theory (DFT) and molecular dynamics (MD) were used in conjunction with differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) to explore the oligomer microstructures, the crystal structure, and the temperature dependence of the specific volume (1/ρ). DFT geometry minimizations on isolated oligomers were used to identify the lowest energy confirmer; revealing that alternating R,R and S,S chiral bonds between monomer units afford the lowest energy structure. MD simulations of crystalline PCHD were constructed so as to replicate the experimental XRD pattern of crystalline PCHD, with the best fit producing a monoclinic crystal structure. The temperature dependence of the specific volume derived from MD simulations provided insight into the glass/vitrification (T_g) and melting (T_m) transition temperatures. Comparison of the simulation transition temperatures with differential scanning calorimetry data of PCHD polymerized with Ni(acac)₂/MAO shows good agreement and solidifies the fidelity of the newly defined PCHD crystalline structure.     

Characterization and thermal‐oxidative degradation behavior of poly(ethylene terephthalate‐co‐4,4′‐bibenzoate) prepared via direct esterification technique

A series of poly(ethylene terephthalate‐co‐4,4′‐bibenzoate)s (PETBBs) were prepared via direct esterification from the monomers of terephthalic acid (TPA), 4,4′‐biphenyl dicarboxylic acid (BPDA), and ethylene glycol (EG) with different molar ratios. The chemical compositions of the obtained PETBBs, investigated by H¹‐NMR, were identical with the feed ratio, and the high molecular weights of PETBBs were confirmed by GPC analysis. The glass transition, crystallization, and melting behavior of them were measured by DSC; the results indicated that, in the range of 5–25 mol% of BPDA addition, the glass transition temperature (T_g) increased almost linearly and the melting temperature (T_m) decreased with increasing content of BPDA unit. As expected, the crystallization of PETBB became difficult with increasing introduction of BPDA, explained by higher crystallization temperature and smaller crystallization enthalpy from the glassy state. This decrease of crystallization rate may be beneficial to film processing. Moreover, owing to the introduction of rigid‐rod BPDA unit, the initial and maximum thermal‐oxidative decomposition temperatures were enhanced. The kinetic analysis of the thermal‐oxidative degradation indicated that the apparent activation energies of degradation for these PETBBs became higher than that of PET. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Synthesis and thermotropic liquid‐crystalline behaviour of novel main‐chain poly(ether ether ketone ketone)s containing a lateral tert‐butyl group

The synthesis of novel poly(ether ether ketone ketone)s containing a lateral group via the random copolymerization of 4,4′‐biphenol, tert‐butylhydroquinone and 1,4‐bis(p‐fluorobenzoyl)benzene is described. The copolymers were characterized by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM) observation. The results showed that the thermotropic liquid‐crystalline properties were achieved in the copolymers containing 30 mol% and 50 mol% tert‐butylhydroquinone, which have relatively lower melting temperatures due to the copolymerization effect. Both the crystalline–liquid‐crystalline transition (T_m) and the liquid‐crystalline–isotropic phase transition (T_i) were observable in the DSC thermograms, while the biphenol‐based poly(aryl ether ketone) has only one melting transition. The hydroquinone‐based polymer was shown to be amorphous. Thermogravimetric analysis (TGA) results showed that these copolymers are all high‐temperature resistant with higher glass transition temperature between 147 and 149 °C, and higher decomposition temperature T_d in the range 480–520 °C. © 2000 Society of Chemical Industry     

Biodegradable poly(ethylene succinate) blends and copolymers containing minor amounts of poly(butylene succinate)

Poly(ethylene succinate) (PES), poly(butylene succinate) (PBS), and PES‐rich copolyesters were synthesized using an effective catalyst, titanium tetraisopropoxide. PES was blended with minor amounts of PBS for the comparison. The compositions of the copolyesters and the blends were determined from NMR spectra. Their thermal properties were studied using a differential scanning calorimeter (DSC), a temperature modulated DSC (TMDSC), and a thermogravimetric analyzer. No significant difference exists among the thermal stabilities of these polyesters and blends. For the blends, the reversible curves of TMDSC showed a distinct glass‐rubber transition temperature (T_g), however, the variation of the T_g values with the blend compositions was small. Isothermal crystallization kinetics and the melting behavior after crystallization were examined using DSC. Wide‐angle X‐ray diffractograms (WAXD) were obtained for the isothermally crystallized specimens. The results of DSC and WAXD indicate that the blends have a higher degree of crystallinity and a higher melting temperature than those of the corresponding copolymers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Block copolyetheresters. V. Low-temperature properties of thermotropic block copolyetheresters

The low-temperature properties of block copolyetheresters with hard segments of poly(alkylene p,p′-bibenzoate) and soft segments of poly(tetramethylene ether) were investigated by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In the temperature range of −100 to 60°C, two transition temperatures, a glass transition temperature (T_g) and a melting temperature (T_m), were found by DSC and are attributed to the polyether segments. The T_g monitored by DSC of the polyether segments of the block copolyetheresters is around −68°C and independent of the composition and the type of polyester segment. Thus, the amorphous parts of the polyether segments should be immiscible with the amorphous parts of the polyester segments. The polyether segments of the block copolyetheresters exhibit a lower T_m and a lower crystallinity than those of the poly(tetramethylene ether)glycol due to the presence of the polyester segments. The crystallizability of the polyether segments is dependent on the composition to some extent. The DMA data show that the dynamic modulus drops more abruptly around −10 to 15°C, indicating that the mechanical properties may change significantly due to the melting of the polyether segments. © 1996 John Wiley & Sons, Inc.     

Evaluation of local polarity of polymer solids by a rigid fluorescent probe of carbazole-terephthalate cyclophane

A novel rigid fluorescent probe, carbazole-terephthalate cyclophane (Cz-TP) was applied to evaluate local dielectric constants (ε) of various polymer solids in a wide range of temperatures. For poly(vinylidene fluoride), the ε increased above the glass transition temperature (T_g), due to relaxations of the polar segment -(CH₂CF₂)- of the main chain. For poly(alkyl methacrylate)s, the ε increased above the T_g or the melting temperature of the side chain, where motions of the polar ester groups are activated. For cyanoethylated polymers, the ε increased owing to motions of the polar cyano groups at the end of the side chain and the ε corresponded to the dielectric constant evaluated by dielectric relaxation measurement at a high frequency, because the Cz-TP exciplex has a lifetime of tens of nanoseconds. For a cyanoethylated polymer with a high content of cyano groups, the ε was larger at low temperatures than the dielectric constant obtained by the macroscopic dielectric relaxation measurement. These results show that the Cz-TP molecule is a useful probe for evaluation of the local polarity in polymer solids over a wide temperature range and can detect even a small change in ε at transition temperatures such as glass transition, side-chain melting, and side-chain relaxation.     

Molecular dynamics in precision deuteriomethyl branched polyethylene from solid-state deuterium NMR

Deuterium quadrupolar echo NMR was applied to precision CD₃ branched polyethylene at temperatures ranging from below the glass transition up to the melting point. The CD₃ branches were placed on every 15th or 21st carbon with zero variation in the branch spacing by acyclic diene metathesis polymerization chemistry. The deuterium lineshapes were simulated and fit to the experimental spectrum assuming appropriate models that approximate the motions in the amorphous and crystalline phases. Spectral contributions of each phase were isolated by T₁ fitting. The fitting results comprise the isotropic reorientation correlation time distribution and axial jump angle distribution in these two phases, respectively. The mean jump angle was found to increase monotonically with temperature, approaching 35° near the melting point, consistent with previous carbon-13 NMR results on this same polymer.     

Microstructural effects on hot drawing syndiotactic styrene P‐methyl styrene copolymer

Crystalline syndiotactic styrene/p‐methyl styrene copolymer (SPMS) has been oriented by tensile drawing at various temperatures between the glass transition and crystalline melting point. The microstructural changes resulting from drawing have been studied using differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). WIth increasing draw temperature, both melting temperature and crystalline dimensions of the oriented samples increase. The heat of fusion increases with increasing draw temperature up to ～200°C. It also increases with draw ratio and draw rate, while the crystalline width increases only with draw ratio. THe amorphous fraction shows a clear glass transition, the temperature of which (T_g) increases with draw ratio. However, T_g decreases somewhat with increasing draw temperature. This is interpreted in terms of the stretching of the randomly coiled amorphous phase molecules.     

Comparison of thermal techniques for glass transition measurements of polystyrene and cross-linked acrylic polyurethane films

There are few quantitative comparisons in the literature between glass transitions (T_g) measured by differential scanning calorimetry (DSC) and by dynamic mechanical analysis (DMA). Also, in the case of DMA, two different operational definitions have been used to obtain the glass transition, namely, the loss modulus (E″) and damping (tan δ) peak temperatures. We propose a new DMA definition of T_g and demonstrate that it agrees with DSC T_g measurements within ±2°C for both thermoplastic polystyrene and thermoset cross-linked acrylic polyurethane films with measurable tan δ peaks. The glass transitions for a single polystyrene standard and several cross-linked acrylic polyurethane films were measured by DSC. Additionally, E″ and tan δ peak temperatures were measured by DMA as a function of frequency and temperature. Empirically, it was determined that the average of the E″ and tan δ peak temperatures measured at 1 rad/s oscillation frequency corresponds to the glass transition measured by the ASTM E1356 DSC test method. © 1994 John Wiley & Sons, Inc.     

Poly(trimethylene terephthalate) copolyester containing ethylene glycol moieties. Part 1: Crystallization kinetics and melting behavior

A copolyester was characterized to have 91 mol% trimethylene terephthalate unit and 9 mol% ethylene terephthalate unit in a random sequence by using ¹³C NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 180 to 207 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC). The exothermic behavior in the DSC and TMDSC curves gives a direct evidence of recrystallization. No exothermic flow and fused double melting peaks at T_c = 204 °C support the mechanism of different morphologies. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 236.3 °C. The kinetic analysis of the growth rates of spherulites and the morphology change from regular to banded spherulites indicated that there existed a regime II → III transition at 196 °C.     

Self-Healing Ability of Poly(PEGMA-5-UPy) Evaluated by Thermomechanical Analysis

The synthesis and characterization of polyethylene glycol monomethyl ether methacrylate (PEGMA) based copolymers incorporating three different percentages (2.5 wt%, 5.0 wt%, and 7.8 wt%) of urea-N-2-amino-4-hydroxy-6-methylpyrimidine-N’-(hexametylen-n-carboxyethyl methacrylate) (HEMA-UPy) are reported. Nuclear magnetic resonance (NMR) and infrared spectroscopy (IR) confirm the synthesis procedure. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are employed to evaluate the thermal properties of the samples. DSC measurements evidence a slight increase in glass transition temperature (T_g), a consistent increase in crystallization and melting temperatures (T_c and T_m), and a reduction in the crystallization degree (X_c) with increasing the amount of HEMA-UPy moiety. Dynamic mechanical analysis (DMA) is carried out at different values of temperature and oscillation frequency. It highlights the ability of the healed copolymer to recover the pristine values of storage modulus. The healing efficiency depends on the temperature history of the sample. For the sample healed at room temperature, the value of healing efficiency is 64%. DMA tests performed at higher temperatures, after some permanence at room temperature, evidence higher values in the healing efficiency. This demonstrates that the higher value of the temperature employed during DMA tests determines greater mobility of the chains causing an enhancement in the healing efficiency.     

Synthesis and properties of novel poly(tetramethylsilnaphthylenesiloxane) derivatives

Novel poly(tetramethylsilnaphthylenesiloxane) derivatives were synthesized and characterized by differential scanning calorimetry (DSC), thermogravimetry (TG), and X-ray diffraction analyses. Poly(tetramethylsilnaphthylenesiloxane) derivatives were obtained by condensation polymerization of the corresponding disilanol derivatives, i.e. 1,4-, 1,5-, 2,6-, and 2,7-bis(dimethylhydroxysilyl)naphthalenes, which were prepared by the Grignard reaction using chlorodimethylsilane and the corresponding dibromonaphthalene derivatives followed by the hydrolyses, catalyzed by palladium on charcoal. The obtained poly(tetramethyl-1,5-silnaphthylenesiloxane) was insoluble in common organic solvents; however, the other polymers exhibited the good solubility in common organic solvents, such as tetrahydrofuran (THF), chloroform, dichloromethane, and toluene. The introduction of tetramethyl-1,5-silnaphthylenesiloxane units into the resulting polymer was confirmed by ¹H NMR spectrum of the copolymer obtained by condensation copolymerization of 1,5-bis(dimethylhydroxysilyl)naphthalene with 1,4-bis(dimethylhydroxysilyl)naphthalene. It was revealed from the DSC and X-ray diffraction measurements that poly(tetramethyl-1,5-silnaphthylenesiloxane) and poly(tetramethyl-2,6-silnaphthylenesiloxane) exhibited the crystallinity; however, poly(tetramethyl-1,4-silnaphthylenesiloxane) and poly(tetramethyl-2,7-silnaphthylenesiloxane) were amorphous. The glass transition temperature (T_g) and the temperature at 5% weight loss (T_d5) of poly(tetramethylsilnaphthylenesiloxane) derivatives with dimethylsilyl group at 1-position of the naphthylene moiety were higher than those at 2-position of the naphthylene moiety. The T_g and melting point (T_m) of the present polymers were higher than those of poly(tetramethyl-1,4-silphenylenesiloxane).     

Fast physical drying,high water and salt resistant coatings from non-drying vegetable oil

This paper reports fast physical drying, high water and salt resistances of coating materials from non-drying palm oleic acid. Short oil-length alkyd was synthesized and copolymerized with methyl methacrylate. Three copolymers of the alkyd and methyl methacrylate with different alkyd/MMA ratios were prepared via free radical polymerization. The copolymers were characterized by FTIR and H NMR spectroscopy, and glass transition temperatures (T_g) were measured by DSC. The decreasing amount of alkyd was noticed to increasing conversion and T_g. The overall thermal stability has increased with higher amount of alkyd in the copolymer. Moreover, incorporation of alkyd has improved the adhesion and film hardness of the coatings.