Radiation-induced solid-state polymerization in binary systems. IV. Solid-state polymerization in the glassy phase

The radiation-induced polymerization of glass-forming systems containing monomers has been investigated. It was found that irradiation below the second-order transition temperature T_g of the systems causes no in-source polymerization but causes a rapid postpolymerization on warming above the T_g after initial irradiation below the T_g. The post-polymerization was followed by differential thermal analysis and ESR spectra. It is caused above the T_g by the release of peroxy radicals trapped below the T_g, and its rate is proportional to the irradiation dose to some extent, often is explosively high, and brings about a remarkably large temperature rise by accumulation of polymerization heat. Irradiation above the T_g causes rapid in-source polymerization which is accelerated by the high viscosity of the monomeric system between T_g and T_s (WLF temperature) compared to crystal or ordinary solution polymerization. The temperature dependence of the in-source polymerization of glassy systems shows a peak between the T_g and T_s which may be the result of competing effects of the rate increase by the decreased termination near T_s and the rate decrease by the decreased propagation caused by the diffusion prevented near the T_g. The degree of polymerization was also investigated. The temperature dependence of the degree of polymerization of the polymers obtained by in-source polymerization shows a peak similar to that of the temperature dependence of conversion. Unusually large values of the Huggins constant k' are noted between T_g and T_s. The degree of polymerization of the polymer obtained by post-polymerized increases with the increase of irradiation dose and the polymerization rate; this may be the result of decreased chain transfer to nonpolymerizable components.     

Radiation-induced polymerization of glass-forming systems. V. Initial polymerization rate in binary glass-forming systems

The radiation-induced polymerization of binary systems consisting of glass-forming monomer and glass-forming solvent in supercooled phase was studied. The initial polymerization rates were markedly affected by T_g (glass transition temperature) and T_v of the system (30–50°C higher than T_g), which turned to be functions of the composition. The composition and temperature dependence of initial polymerization rate in binary glass-forming systems were much affected by homogeneity of the polymerization system and the T_g of the glass-forming solvent. The composition and temperature dependences in the glycidyl methacrylate–triacetin system as a typical homogeneous polymerization system were studied in detail, and the polymerizations of hydroxyethyl methacrylate–triacetin and hydroxyethyl methacrylate–isoamyl acetate systems were studied for the heterogeneous polymerization systems; the former illustrates the combination of lower T_g monomer and higher T_g solvent and the latter typifies a system consisting of higher T_g monomer and lower T_g solvent. All experimental results for the composition and temperature dependence of initial polymerization rate in binary glass-forming systems could be explained by considering the product of the effect of the physical effect relating to T_v and T_g of the system and the effect of composition in normal solution polymerization at higher temperature, which was also the product of a dilution effect and a chemical or physical acceleration effect.     

Impact of bottlebrush chain architecture on Tg-confinement and fragility-confinement effects enabled by thermo-cleavable bottlebrush polymers synthesized by radical coupling and atom transfer radical polymerization

Thermo-cleavable bottlebrush polymers were synthesized by a facile grafting-to method via radical coupling and atom transfer radical polymerization (ATRP) without small-molecule synthesis involved. Bottlebrushes were achieved by coupling backbones of poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl), which contain nitroxide radicals, and ATRP-synthesized side chains, which can be halogen-abstracted to generate carbon-centered radicals. Bottlebrushes were prepared using homopolymer or block copolymer side chains. Alkoxyamine covalent bonds resulting from radical coupling are thermo-reversible at high temperature, and grafting density may be tuned by annealing of post-synthesis bottlebrush, with the bottlebrush regime going from loose bottlebrush to dense comb and loose comb. The effects of confinement on the T_g and fragility of films of polystyrene bottlebrush were studied by ellipsometry; comparisons were made to thermally cleaved linear components obtained directly after annealing. Relative to linear polymer, bottlebrush topology reduces bulk fragility and suppresses T_g- and fragility-confinement effects. The correlation between the strengths of the confinement effects is consistent with other film studies of linear and non-linear polymers and supports the notion that fragility is a fundamental property underlying perturbations to T_g. Besides providing a platform for advancing fundamental scientific understanding, our synthetic strategy may afford novel applications of bottlebrushes via incorporated dynamic chemistry.     

The application of simultaneous chemiluminescence and thermal analysis for studying the glass transition and oxidative stability of poly(N-vinyl-2-pyrrolidone)

A computer-controlled chemiluminescence (CL) instrument incorporating a differential scanning calorimeter was used to simultaneously acquire photochemical and enthalpic data for poly(N-vinyl-2-pyrrolidone) (PVP). Samples were subjected to a linear temperature ramp under nitrogen and their luminescence response recorded. The resultant dynamic CL peak is attributable to the decomposition of hydroperoxide groups and the subsequent mutual termination of secondary polymer peroxyl radicals. It is shown that dynamic CL can be used to characterize the level of oxidation in PVP, which in commercial samples, may be partly related to the level of residual polymerization inititor and to the drying process. The temperature at which maximum CL emission occurs correlates with the glass transition temperature (T_g) of the polymer and increases with increasing molecular weight. A marked increase in the T_g of PVP occurs after it is aged in air for 24 h at 120°C. This is due to the loss of adsorbed moisture from the polymer which was confirmed by thermogravimetric analysis. Oxidation profiles of PVP were obtained by plotting the integrated CL peak area as a function of aging time. The profiles are compared with data obtained from isothermal CL and viscosity measurements. Gas perturbation experiments suggest that when drying PVP under nitrogen at elevated temperatures significant populations of longlived macroalkyl radicals are formed which can peroxidize the polymer on exposure to air. © 1993 John Wiley & Sons, Inc.     

Radiation-induced polymerization of glass-forming systems. II. Effect of temperature on the polymerization rate at higher conversion

The effect of temperature and conversion on the polymerization rate at higher conversion was investigated with regard to the γ-ray-induced polymerization of hydroxyethyl methacrylate (HEMA) and glycidyl methacrylate (GMA) in the supercooled phase. The polymerization rate changed from acceleration to depression at various conversions, depending on the polymerization temperature. It was found that T_v at which the viscosity of the system became ca. 10³ cpoise influenced the shape of the polymerization time–conversion curve. The experimentally obtained conversion reflection point in the polymerization time–conversion curve agreed with the conversion where the polymerization temperature is the same as the calculated T_v of the system. When the polymerization temperature was lower than T_v of the monomer, no acceleration of the polymerization occurred. When the polymerization temperature was higher than T_v of the polymer, no depression of the polymerization rate was observed. The effect of temperature on the saturated conversion (final conversion) was also examined in terms of T_g of the polymerization system. The experimentally obtained saturated conversion agreed with the conversion where the polymerization temperature is the same as the calculated T_g of the system.     

Radiation-Induced Solid-State Polymerization in Binary Systems. VI. Polymerization of Vinyl Compounds in the Supercooled Liquid Phase

Abstract

The radiation-induced polymerization of glass-forming systems containing vinyl monomers was investigated. Irradiation below the secondorder transition temperature (T_g) of the systems causes no in-source polymerization but does cause a very rapid postpolymerization in the course of heating above T_g. Differential thermal analysis was carried out to estimate T_g and to follow the postpolymerization.     

Radiation-induced polymerization of glass forming systems. VI. Polymerization rate at higher conversion in binary systems

The effect of temperature and composition on the inflection point in the time–conversion curve and the saturated conversion was investigated in the radiation-induced radical polymerization of binary systems consisting of a glass-forming monomer and a solvent. In the polymerization of completely homogeneous systems such as glycidyl methacrylate (GMA)–triacetin and hydroxyethyl methacrylate (HEMA)–propylene glycol systems, the time–conversion curve has an inflection point at polymerization temperatures between T_vm (T_v of monomer system) and T_vp (T_v of polymer system). Such conversions at the inflection point changed monotonically between 0 and 100% in this temperature range. T_v was found to be 30–50°C higher than T_g (glass transition temperature) and a monotonic function of composition (monomer–polymer–solvent). The acceleration effect continued to 100% conversion above T_vp, and no acceleration effect was observed below T_vm. The saturated conversion in homogeneous systems changed monotonically between 0 and 100% for polymerization temperatures between T_gm (T_g of monomer system) and T_gp (T_g of polymer system). T_g was also a monotonic function of composition. No saturation in conversion was observed above T_gp, and no polymerization occurred below T_gm. In the polymerization of completely heterogeneous systems such as HEMA–dioctyl phthalate, no acceleration effect was observed at any temperature and composition. The saturated conversion was 100% above T_g of pure HEMA, and no polymerization occurred below this temperature in this system.     

Glass and melting transitions of copolymers of tetrafluoroethylene with propylene and isobutylene

Copolymers of tetrafluoroethylene and propylene were prepared that contained 30–65 mole-% of the former. Reactivity ratios of tetrafluoroethylene- and propylene-ended radicals are 0.008 and 0.06, respectively, resulting in formation of highly alternating copolymers. The glass temperatures, T_g, were determined using a differential scanning calorimeter. Values ranged from 260 to 275°K. A plot of T_g versus composition has a low maximum centered about the equimolar composition. Copolymers of tetrafluoroethylene and isobutylene were prepared that contained 30–56 mole-% of the former. Reactivity ratios of tetrafluoroethylene- and isobutylene-ended radicals are 0.005 and 0.021, respectively. The glass temperatures of these copolymers range from 257 to 313°K. A higher maximum at the equimolar composition is obtained when T_g is plotted versus composition. Isobutylene-containing copolymers having 45–54 mole-% tetrafluoroethylene are crystalline. Melting temperatures range from 416 to 476°K and have their maximum value at the equimolar composition. It is thought that long sequences of alternating units behave as a third entity in these copolymers, the other two being nonalternating units of the two monomers. Unless inhibited, ionic homopolymerization of isobutylene can be appreciable, sometimes resulting in the polymer having two T_g.     

Polymerisation radiochimique de l'acide acrylique en solution dans l'alcool isopropylique et le dimethylformamide

The radiation-induced polymerization of acrylic acid in isopropyl alcohol and dimethylformamide solutions was investigated between?196°and40°. Mixtures which form glasses at low temperatures exhibit a maximum in the rate of polymerization at 30° and 50° above the glass transition temperature (T_g). The difference between the most favourable temperature for polymerization and T_g is larger than in systems studied previously. This fact could be due to the presence of H-bonded aggregates.The study of the polymerization of acrylic acid in dimethylformamide solution at 20° led to a correlation between this reaction and the presence of plurimolecular aggregates. The very high polymerization rate and the syndiotacticity of the resulting poly(acrylic acid) confirm the earlier assumption of a favourable orientation for propagation of the molecules of acrylic acid in these aggregates.     

Greffage radiochimique de l'acrylonitrile sur le poly(methacrylate de methyle)

The radiation-induced polymerization of acrylonitrile with dissolved PMMA exhibits kinetics similar to those found with the pure monomer. The addition of PMMA to the monomer at first leads to an increase in polymerization rate; a maximum in rate is observed for 60 per cent acrylonitrile in the mixture. The unreacted PMMA was quantitatively extracted by toluene from the reaction mixture. In contrast, polyacrylonitrile could not be separated from the graft copolymer by fractional precipitation, presumably due to association of the graft copolymer with the precipitated homopolymer. The free radical yield of PMMA “G_R effective” derived from these results was found to be 8 to 10 in mixtures containing small amounts of monomer. It rapidly decreased as the monomer concentration increased.The solubilities of the graft copolymers were characterized by the precipitation γ determined for several precipitants in DMF solutions. A maximum in solubility was found for copolymers containing 25 to 35 per cent acrylonitrile in DMF-alcohol mixtures. The glass transition temperatures (T_g) of the graft copolymers were measured using a penetrometer. T_g increased with the MMA content in the copolymer. A small minimum of T_g appeared to exist for copolymers containing 90 per cent acrylonitrile.     

Functional Polyolefins: Poly(ethylene)‐graft‐Poly(tert‐butyl acrylate) via Atom Transfer Radical Polymerization From a Polybrominated Alkane

Poly(cis‐cyclooctene) is synthesized via ring‐opening metathesis polymerization in the presence of a chain‐transfer agent and quantitatively hydrobrominated. Subsequent graft polymerization of tert‐butyl acrylate (tBA) via Cu‐catalyzed atom transfer radical polymerization (ATRP) from the non‐activated secondary alkyl bromide moieties finally results in PE‐g‐PtBA copolymer brushes. By varying the reaction conditions, a series of well‐defined graft copolymers with different graft densities and graft lengths are prepared. The maximum extent of grafting in terms of bromoalkyl groups involved is approximately 80 mol%. DSC measurements on the obtained graft copolymers reveal a decrease in T_m with increasing grafting density.     

Structural investigation of polystyrene grafted and sulfonated poly(tetrafluoroethylene) membranes

Structural investigation of polystyrene grafted and sulfonated poly(tetrafluoroethylene) (PTFE) membranes prepared by radiation-induced grafting of styrene onto commercial PTFE films and subsequent sulfonation was carried out by differential scanning calorimetry and X-ray diffraction. The effect of the structural changes taking place in the membranes during the preparation procedure (grafting and sulfonation) and the variation of the degree of grafting on melting temperature (T_m), glass transition temperature (T_g), heat of melting (ΔH_m), and degree of crystallinity was studied. The melting temperature (T_m) was found to be independent of the degree of grafting unlike glass transition temperature (T_g), which was found to be a function of the degree of grafting. Moreover, the degree of crystallinity of the membranes was found to decrease with the increase in the degree of grafting. The results of this work suggest that grafting takes place in the entire amorphous region without any significant disruption in the crystalline structure of PTFE film and the decrease in the degree of crystallinity is mainly attributed to the dilution effect.     

Dynamics and Crystallization in Polydimethylsiloxane Nanocomposites

Several routes were used to achieve silicon nanocomposites. The first and second one are the melt intercalation of polydimethylsiloxane (PDMS), which is a mechanical blending of the polymer in the molten state with the untreated inorganic filler or intercalated nanoparticles. The last one is an in situ polymerization, which previously requires the intercalation of hexamethylcyclotrisiloxane (D₃) followed by a subsequent polymerization step. We used synthetic mineral oxide HTiNbO₅ as nanofiller. These systems were investigated by differential scanning calorimetry (DSC) and solid state NMR in order to better understand the relation between the nanocomposites dynamics, and crystallisation. The efficiency of grafting reactions was studied by ²⁹Si CP/MAS NMR. The nature of the interfacial interactions seems to play the major role. Indeed, the nanocomposites 1 and 2 for which only physical interactions are expected do not exhibit any T_g deviation whereas the nanocomposite 3, for which chemical grafting is achieved, increases strongly the T_g. Crystallization is more sensitive to density and strength of interfacial interactions which are maximum for the pristine filler.     

Radical polymerization of styrene in the presence of C60

Radical polymerizations of styrene in the presence of C₆₀ have been conducted at 90°C in benzene using benzoyl peroxide (BPO) as initiator. The behaviors of C₆₀ are investigated by monitoring BPO concentration, C₆₀ content, and polymerization time. It is found that C₆₀ acts like a radical absorber which multiply absorbs primary radicals from BPO and propagating radicals. Therefore, in the presence of C the yield and molecular weight decrease significantly. However, the molecular weight distribution is narrowed down by its coupling characteristics. At the beginning of the reaction, owing to the radical-absorbing effect of C₆₀, it makes the chain-propagation restricted. However, the number of polystyrene chains added to C₆₀ increases with polymerization time. Direct dilatometric experiment proves that C₆₀ is mainly as inhibitor for radical polymerization of styrene by benzoyl peroxide. Besides, the glass transition temperature (T_g) of the copolymers increases with increasing content of C₆₀. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2969–2975, 1999     

Glass‐Transition Temperature as a Function of Conversion in Hyperbranched Polymers Obtained by Self‐Condensing Vinyl Polymerization

Chain‐end free volume theory is extended for studying the glass‐transition temperature (T_g) as a function of conversion in hyperbranched polymers. T_g is found to have a non‐linear inverse relationship to the molecular weight for polymers obtained by self‐condensing vinyl polymerization (SCVP). During the monomer conversion process, T_g decreases with the increase in molecular weight (P) in the low conversion range, then levels off in the high conversion range.     

Synthesis of poly(2‐furyloxirane) with high molecular weight and improved regioregularity using macrocyclic ether as a cocatalyst to potassium tert‐butoxide

2‐Furyloxirane (FO), a monomer usually obtained from a nonpetroleum route, was prepared from the epoxidation reaction of furfural and trimethylsulfonium chloride. About 200–300 g FO can be obtained in each preparation process. Although anionic polymerization of FO generally gives low‐ molecular‐weight polymers even after long periods of polymerization, the reaction was greatly improved when macrocyclic ether was used as a cocatalyst to potassium tert‐butoxide. When 18‐crown‐6 was used as a cocatalyst, poly(2‐furyloxirane) (PFO) with a number‐average molecular weight (M_n) of 41.5 kg/mol and a polydispersity index of 1.3 was obtained at 94% yield after polymerization at 40 °C for 72 h. The PFO obtained contained a 61.7% head‐to‐tail (H‐T) structure in the absence of the macrocyclic ether, and it reached 70.6% when cryptand[2,2,2] was used as a cocatalyst. PFO with higher regioregular structures showed improved thermal properties. For PFO with M_n of around 20.0 kg/mol, its glass transition temperature (T_g) increased from ?3 to 6 °C when the H‐T content was increased from 61.7 to 70.6%. Raising the M_n of PFO also raised T_g. For PFO with 68.9% H‐T structure, its T_g could reach 7 °C when M_n was increased to 40 kg/mol. This study shows two effective ways to improve the thermal and mechanical performances of the polymer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Structural relaxation of stacked ultrathin polystyrene films

The T_g depression and kinetic behavior of stacked polystyrene ultrathin films is investigated by differential scanning calorimetry (DSC) and compared with the behavior of bulk polystyrene. The fictive temperature (T_f) was measured as a function of cooling rate and as a function of aging time for aging temperatures below the nominal glass transition temperature (T_g). The stacked ultrathin films show enthalpy overshoots in DSC heating scans which are reduced in height but occur over a broader temperature range relative to the bulk response for a given change in fictive temperature. The cooling rate dependence of the limiting fictive temperature, T_f′, is also found to be higher for the stacked ultrathin film samples; the result is that the magnitude of the T_g depression between the ultrathin film sample and the bulk is inversely related to the cooling rate. We also find that the rate of physical aging of the stacked ultrathin films is comparable with the bulk when aging is performed at the same distance from T_g; however, when conducted at the same aging temperature, the ultrathin film samples show accelerated physical aging, that is, a shorter time is required to reach equilibrium for the thin films due to their depressed T_g values. The smaller distance from T_g also results in a reduced logarithmic aging rate for the thin films compared with the bulk, although this is not indicative of longer relaxation times. The DSC heating curves obtained as a function of cooling rate and aging history are modeled using the Tool-Narayanaswamy-Moynihan model of structural recovery; the stacked ultrathin film samples show lower β values than the bulk, consistent with a broader distribution of relaxation times. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2741–2753, 2008     

Breadth of glass transition temperature in styrene/acrylic acid block,random, and gradient copolymers: Unusual sequence distribution effects

Block, random, and gradient copolymers of styrene (S) and acrylic acid (AA) are synthesized by conventional or controlled radical polymerization, and their glass transition temperature (T_g) behaviors are compared. The location and breadth of the T_gs are determined using derivatives of differential scanning calorimetry heating curves. Each S/AA random copolymer exhibits one narrow T_g, consistent with a single phase of limited compositional nanoheterogeneity. Block copolymers exhibit two narrow T_gs originating from nanophase separation into ordered domains with nearly pure S or nearly pure AA repeat units. Each gradient copolymer exhibits a T_g response with a ～50–56 °C breadth that extends beyond the upper T_g of the block copolymers. For copolymers of similar composition, the maximum value in the gradient copolymer T_g response is consistent with that of a random copolymer, which has an enhanced T_g relative to poly(acrylic acid) due to more effective hydrogen bonding when AA units are separated along the chain backbone by S units. These results indicate that gradient copolymers with ordered nanostructures can be rationally designed, which exhibit broad glass transitions that extend to higher temperature than the T_gs observed with block copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2842–2849, 2007     

Radiation-induced polymerization of glass-forming systems. VII. Polymerization in supercooled state under high pressure

Radiation-induced polymerization of glass-forming monomers such as 2-hydroxyethyl methacrylate and glycidyl methacrylate under high pressure was studied. The glass transition temperature of these monomers was heightened by increased pressure. The temperature dependence of polymerizability showed a characteristic relation; similar to those in supercooled-phase polymerization under normal pressure, that had a maximum at T_v which shifted to higher levels of temperature as well as to T_g under high pressure. Polymerizability in the supercooled state also increased under increased pressure.     

Crosslinked liquid crystal polymers from liquid crystal monomers: Synthesis and mechanical properties

Difunctional acrylates and methacrylate monomers have been made which are high order smectic liquid crystal (or crystalline) at room temperature. This report discusses materials with the following structure: F–S–M–S–F, where F is a functional group, acrylate or methacrylate (A or M); S is a spacer (CH₂)_n(n), and M is a mesogen—in this case 4,4′-dioxybiphenyl (B). They are codified as BnA or BnM where n is the number of methylenes in the spacer. High conversion with high T_g can be obtained when polymerizing in the smectic state because the reactive end groups are concentrated in a small volume and can react well with little or no diffusion. B2A, B3A, B6A, B11A, and B3M were polymerized in the smectic state and compared to polymers made at temperatures where the monomers were isotropic. High conversion was obtained below final T_g—even then, probably because the polymers were ordered. All the polymers were studied by WAXD and dynamic mechanical spectroscopy. Solid-state NMR on B3A showed that there was very high conversion of the double bonds at all temperatures. B3A photopolymerized in the smectic state (60–76°C) produced a crystalline polymer with T_g = 185°C (1 Hz). When photopolymerized at 85°C, above the isotropization temperature (T_i), a poorly organized polymer was obtained with a T_g of 155°C (1 Hz). Monomers with an odd number of methylene groups as spacers were crystalline after polymerization. With an even number of methylene groups, they lost most of their crystallinity on polymerization below T_i, but retained a low order smectic structure. Similar structures were obtained with all the monomers when they were polymerized above T_i. There was little effect of polymerization temperature on T_g when the spacers had an even number of methylene groups. © 1993 John Wiley & Sons, Inc.