Prepolymer Molecular Weight Research Articles

Continuous reactor preparation of thermoplastic modified epoxy‐amine prepolymers

AbstractThis paper describes a new continuous reactor method to prepare thermoplastic modified epoxy prepolymers for aerospace prepregs with the aim of replacing traditional batch reactors. Compared with batch reactors, the continuous reactor is capable of producing epoxy prepolymers through simultaneous dissolution of polyethersulfone (PES) and 4,4′‐diaminodiphenylsulfone in tetraglycidyl‐4,4′‐ diaminodiphenylmethane (TGDDM). In addition, concurrent chain extension reactions advance prepolymer molecular weights to desired viscosities in less than 2 min of mean residence time. Optical micrographs were used to define how process temperature influences PES dissolution in TGDDM in a continuous reactor. Kinetic studies confirmed that the chain extension reaction in a continuous reactor is similar to that in a batch reactor, and the molecular weights and viscosities of prepolymers were readily controlled through reaction kinetics. Atomic force microscopy was used to confirm similar cured network morphologies for formulations prepared from batch and continuous reactors. Additionally tensile strength, tensile modulus and fracture toughness analyses concluded that mechanical properties of cured epoxy matrices produced from the two reactors were equivalent. © 2014 Society of Chemical Industry

Reversible actuation in main-chain liquid crystalline elastomers with varying crosslink densities

Main-chain smectic-C liquid crystalline elastomers (LCEs) with varying crosslink densities were prepared using a two stage hydrosilylation reaction scheme, in which diene mesogens were first polymerized with hydride-terminated poly(dimethylsiloxane) and subsequently crosslinked using a tetravinyl molecule. Adjustment of prepolymer molecular weight afforded control over network crosslink density. The LCEs exhibit two thermal transitions that are determined by the mesogen composition and are independent of crosslink density: the mesogen glass transition (∼−30 °C) and isotropization (∼40 °C). Thermal cycling around the isotropization transition under tensile load leads to reversible extension (cooling) and contraction (heating) of the sample, a phenomenon called two-way shape memory or “actuation”. Actuation strains reached up to 30%, a substantial amount for a polydomain LCE. Creep experiments revealed different dynamics between the smectic and isotropic phases, and comparison to actuation results indicated that actuation involves more than simply a transition between isotropic and smectic rheological behavior. Instead, the phase transition itself plays an important role. Wide-angle X-ray scattering analysis revealed that samples strained to the same level, whether by creep or actuation, showed different orientation levels. This indicates that microstructure is not a unique property of the deformed state and supports the hypothesis that cooling to the liquid crystalline phase under stress is important to achieving the large strains associated with actuation.

Influence of the prepolymer molecular weight and free isocyanate content on the rheology of polyurethane modified bitumens

Isocyanate-based modification is lately gaining acceptance as a successful way to give added value to bitumen, a crude oil refining by-product. In order to study the influence of prepolymer type on the rheological properties of the resulting binders, six prepolymers synthesized from polypropylene-glycols (PPG) with varying molecular weight (between 440 and 2425) and different molar excess of a polymeric MDI (4,4′-diphenylmethane diisocyanate) were used. Two modification procedures, either involving or not water addition were followed. The modification achieved depends on both the selected polyol molecular weight and the excess in MDI (i.e., free isocyanate content), although not in a similar extent. Viscous flow and dynamic oscillatory shear tests, at 60°C, demonstrated a much higher level of bitumen modification by using the prepolymer prepared with the polyol having a molecular weight of 940 and with a free isocyanate content of 17.4wt.%, mainly after addition of water. On the other hand, bitumen nature greatly affects the final rheological properties of these bituminous products. In that sense, modification results much more effective when conducted on bitumen with a well-developed colloidal microstructure.

UltraHigh Molecular Weight Nonlinear Polycarbonates Synthesized in Microlayers

Ultrahigh molecular weight bisphenol A polycarbonate (BPAPC) with nonlinear chain structures has been synthesized by solid-state polymerization in microlayers at 230 °C. It has been found that the molecular weights of the microlayers of low molecular weight prepolymers with thickness ranging from 5 to 35 μm increased rapidly to ultrahigh molecular weight (300,000–600,000 g/mol) without any discoloration. The amorphous prepolymers exhibited much higher reaction rate than the partially crystallized prepolymers. Although the reactive end group mole ratios of the prepolymers prepared by semibatch melt polycondensation deviated significantly from the stoichiometric value, branching and partial cross-linking reactions have been found to occur to rapidly increase the polymer molecular weight. These nonlinear chain structures observed were due to Fries rearrangement, Kolbe-Schmitt rearrangement reactions, and radical-induced scission/cross-linking reactions. The microstructures of the high molecular weight polymers have been analyzed by 13C NMR, 1H NMR, pyrolysis-gas chromatography mass spectrometry, and atomic force microscopy.

Synthesis and characterization of polyisobutylene-b-polyamide multi-block copolymer thermoplastic elastomers

A series of polyisobutylene (PIB)-polyamide (PA) multi-block copolymers was synthesized via an amidation reaction to form novel thermoplastic elastomers (TPEs). A two prepolymer approach was taken in which a difunctional primary-amine-terminated polyisobutylene (NH2-PIB-NH2) was synthesized as the soft block and a difunctional carboxylic acid-terminated polyamide (HO2C-PA-CO2H) was synthesized separately as the hard block. The two prepolymers were reacted in the bulk under nitrogen at 220 °C to form the multiblock copolymer TPEs. Various copolymers were synthesized from combinations of two different PIBs (either 1000 or 2000 g/mol), and four different PAs (PA-11 and PA-6, each at either 1000 or 2000 g/mol). 1H and 13C NMR spectroscopies were used to confirm the structure of the individual prepolymers and the copolymers. Prepolymer molecular weights were determined by gel-permeation chromatography (GPC), end-group titration, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS); the latter also provided confirmation of end-group functionality of prepolymers. For GPC of PA homopolymers and PIB-PA copolymers, trifluoroacetylation of the secondary amide groups of the PA blocks was necessary to obtain solubility of the copolymers in the THF mobile phase. Copolymer number–average molecular weights varied from 8100 to 20,630 g/mol.

Solid-State Polymerization of Poly(trimethylene terephthalate): Reaction Kinetics and Prepolymer Molecular Weight Effects

The reaction kinetics and the effects of prepolymer molecular weight on the solid-state polymerization (SSP) of poly(trimethylene terephthalate) (PTT) were investigated using nitrogen as the sweep fluid. The synthetic conditions were carefully chosen to eliminate the influences of both internal and external diffusion of the reaction byproducts (1,3-propanediol and water), so that the reaction kinetics were controlled by the forward chain extension reaction. Higher forward reaction rate constants were consistently obtained for the SSP of the high-molecular-weight prepolymer, compared to that of the low-molecular-weight prepolymer. The activation energy of the polymerization reaction was determined to be 15–26 kcal/mol, depending on the prepolymer molecular weight. The slower reaction rate for polymerization of the low-molecular-weight prepolymer may be attributed to the inhibition of the chain-end mobility due to the higher crystallinity and larger lamellar thickness of the obtained polymers. In addition, the high concentration of carboxylic end groups in the low-molecular-weight prepolymer may also decrease the reaction rate by preferential transesterification between 3-hydroxyl propyl end groups over the esterification reaction between 3-hydroxyl propyl and carbonyl end groups. High-molecular-weight PTT with an intrinsic viscosity of 2.05 dL/g (that corresponds to a number average molecular weight of 57600 g/mol) can be obtained via the SSP of the high-molecular-weight prepolymer at a relatively low temperature of 190 °C under the conditions in which byproduct diffusion resistance is eliminated.

UV-curable water-borne polyurethane primers for aluminum and polycarbonate interfaces

Water-borne polyurethane (WPU) primers were synthesized from three types of polyol, viz., poly(propylene glycol), poly(tetramethylene glycol), and polycaprolactone diol (PCL) at two prepolymer molecular weights, and were tested for the adhesion between vinyltrimethoxysilane modified aluminum panel and polycarbonate. It was found that chemical hybridizations of Al panel with WPU via sol–gel reaction were crucial to enhance the adhesion. Among three types of polyol, PCL gave the highest adhesion strength, glassy and rubbery moduli, tensile strength, and glass transition temperature. On the other hand, smaller prepolymer molecular weight gave improved adhesion and improved mechanical properties due to the increased crosslink density and cohesive strength.

Direct laser writing of 3D scaffolds for neural tissue engineering applications

This study reports on the production of high-resolution 3D structures of polylactide-based materials via multi-photon polymerization and explores their use as neural tissue engineering scaffolds. To achieve this, a liquid polylactide resin was synthesized in house and rendered photocurable via attaching methacrylate groups to the hydroxyl end groups of the small molecular weight prepolymer. This resin cures easily under UV irradiation, using a mercury lamp, and under femtosecond IR irradiation. The results showed that the photocurable polylactide (PLA) resin can be readily structured via direct laser write (DLW) with a femtosecond Ti:sapphire laser and submicrometer structures can be produced. The maximum resolution achieved is 800 nm. Neuroblastoma cells were grown on thin films of the cured PLA material, and cell viability and proliferation assays revealed good biocompatibility of the material. Additionally, PC12 and NG108-15 neuroblastoma growth on bespoke scaffolds was studied in more detail to assess potential applications for neuronal implants of this material.

A comprehensive single‐particle model for solid‐state polymerization of poly(L‐lactic acid)

AbstractWe propose here, a comprehensive model for the solid‐state polymerization (SSP) of a low to moderate molecular weight (MW) prepolymer of lactic acid, to produce high MW poly(L‐lactic acid) (PLLA). The reactions are rationally assumed to occur only in the amorphous region, and effective concentrations of end groups, vary with crystalinity, Xc, during SSP. We estimate byproduct diffusivities, D, using free volume theory. The effects of various parameters on the SSP of PLLA prepolymer have been examined with respect to the optimum MW, Xc and D. We introduce self‐consistently, scaling factors of ∼ 0.27, in the experimental procedure, to determine via 19F‐NMR, concentrations of the end groups, after converting them to fluorinated ester groups. The relevant reaction rate constants are obtained by fitting to early time data from representative SSP experiments at 150°C, under high vacuum, on PLLA prepolymer powder (i.e., spherical geometry) of number average MW, Mn0 ∼ 10,200 Da, which attains Mn ∼ 150,000 Da, via SSP. The subsequent successful comparison of the model predictions with experimental data throughout the entire SSP duration indicates that the model is comprehensive and accounts for all the relevant phenomena occurring during the SSP to synthesize high MW PLLA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Preparation of epoxy/urethane graft interpenetrating polymer networks and study of mechanical, thermal and morphological properties

AbstractThis paper describes the preparation of Epoxy/Urethane (EP/PU) graft interpenetrating polymer networks (g-IPNs) and investigates the effect of EP/PU weight ratio and urethane's prepolymer molecular weight on the mechanical, morphological and thermal properties of the IPN system. Here, g-IPN was prepared by thorough mixing of an isocyanate-terminated urethane prepolymer with an epoxy resin followed by simultaneous curing of the resins. Polytetra hydrofuranate (PTHF), molecular weights (Mw) 1000, 2000 and 3000 g/gmol, was used to prepare urethane prepolymers. EP/PU weight ratios were 75/25, 50/50, 30/70 and 15/85. Disappearance of epoxide and isocyanate functional groups was followed by Fourier Transform Infrared spectroscopy (FT-IR), showing curing of the resins. Differential Scanning Calorimetry (DSC) was used to investigate the glass transition temperature (Tg) of the IPNs. Thermal Gravimetric Analysis (TGA), Dynamic Mechanical Thermal Analysis (DMTA), tensile measurements and Scanning Electron Microscopy (SEM) were used to study thermal, mechanical and morphological properties of the prepared systems. The best mechanical properties were obtained at EP/PU weight ratio 75/25 which also shows a fine and uniformly dispersed morphology. Moreover, at this ratio, with increasing PTHF Mw in the urethane prepolymer, the mechanical properties were improved whereas a decrease was observed in Tg and thermal degradation temperature of g-IPNs.

Effect of Matrix Crosslink Density, Varied by Stoichiometry and Resin Molecular Weight, on Fracture Behavior of Epoxy Resins

The role of method of changing the matrix cross-link density (XLD) on toughenability of neat and core-shell rubber modified diglycidyl ether of bisphenol A (DGEBA) based epoxy was investigated. Matrix XLD was varied by changing epoxy equivalent weight and using non stoichiometric amounts of hardener. The samples were cured using di amino di phenyl methane (DDM) and were modified with 10wt% of poly butyl acrylate- poly methyl metacrylate core-shell particles. Fracture toughness values of neat and CSR modified epoxy samples increased with increasing the pre-polymer molecular weight but the fracture energy decreased with lowering the XLD via deviating from stoichiometric point. Scanning electron microscopy was used for analyzing the fracture surface of the samples. The SEM results showed that decreasing the XLD via deviating from stoichiometric ratio causes in variation of crack propagation mode from unstable brittle to stable brittle. For analyzing the deformation behavior of epoxy resins DMTA was utilized. DMTA results showed a suppression of β transition relaxation in non stoichiometric systems which caused antiplasticization of non stoichiometric systems however β relaxation motions in networks with different pre-polymer molecular weights did not change. Results suggest that cross-link density can not be used as a general term for describing the fracture behavior of epoxy resins and it would be more reasonable to use matrix ductility for describing the fracture behavior of epoxy resins.

Morphology, hydration, and proton transport in novel sulfonated polyimide–silica nanocomposites

A series of inorganic–organic nanocomposites containing sulfonated polyimides and silica particles were synthesized. Anhydride-terminated pre-polymers were prepared, followed by introduction of reactive silanes. Casting, curing, and acidification routines lead to nanocomposites with significantly different properties compared to parent sulfonated polyimides. The presence of silica was qualitatively confirmed using energy dispersive X-ray spectrometry and studied using solid-state 29Si NMR. Thermogravimetric analysis provided a more quantitative assessment of the inorganic fraction. Electron microscopy, water vapor sorption, and impedance studies were conducted to understand how silica and ion content influences morphology and proton conductivity. Ion clusters were visible following Ag+ staining, and silica nanoparticles were imaged in unstained samples. Silica nanoparticles significantly reduce the solubility of prepared membranes, and promote membrane hydration. For nanocomposites prepared from high molecular weight pre-polymers, silica incorporation promotes conductivity at low relative humidity. However, for nanocomposites made from low molecular weight or high ionic content pre-polymers, silica dilutes the ion concentration and lowers proton conductivity.

Preparation and characterization of biodegradable poly(lactic acid)‐ block‐poly(ε‐caprolactone) multiblock copolymer

AbstractBiodegradable copolymers of poly(lactic acid)‐block‐poly(ε‐caprolactone) (PLA‐b‐PCL) were successfully prepared by two steps. In the first step, lactic acid monomer is oligomerized to low molecular weight prepolymer and copolymerized with the (ε‐caprolactone) diol to prepolymer, and then the molecular weight is raised by joining prepolymer chains together using 1,6‐hexamethylene diisocyanate (HDI) as the chain extender. The polymer was carefully characterized by using 1H‐NMR analysis, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The results of 1H‐NMR and TGA indicate PLA‐b‐PCL prepolymer with number average molecular weights (Mn) of 4000–6000 were obtained. When PCL‐diols are 10 wt%, copolymer is better for chain extension reaction to obtain the polymer with high molecular weight. After chain extension, the weight average molecular weight can reach 250,000 g/mol, as determined by GPC, when the molar ratio of –NCO to –OH was 3:1. DSC curve showed that the degree of crystallization of PLA–PCL copolymer was low, even became amorphous after chain extended reaction. The product exhibits superior mechanical properties with elongation at break above 297% that is much higher than that of PLA chain extended products. Copyright © 2009 John Wiley & Sons, Ltd.

Complex Effects of the Sweep Fluid on Solid-State Polymerization: Poly(bisphenol A carbonate) in Supercritical Carbon Dioxide

The effects of the sweep fluid on solid-state polymerization (SSP) of poly(bisphenol A carbonate) (BPA-PC) were investigated. Prepolymers with two different number-average molecular weights, PCP6C (Mn = 3800 g/mol) and PCP9C (Mn = 2400 g/mol), were synthesized using melt transesterification. Solid-state polymerization of these prepolymers was carried out at temperatures in the range of 150−190 °C with supercritical carbon dioxide (scCO2) and N2 as the sweep fluids. It was found that scCO2 at 207 bar could either increase or decrease the rate of SSP relative to the rate in atmospheric N2, depending on the prepolymer molecular weight. At 190 °C, the molecular weights of the polymers synthesized from the higher-molecular-weight prepolymer (PCP6C) were higher with scCO2 as the sweep fluid compared to those of the polymers synthesized with N2. In contrast, at the same temperature of 190 °C, the molecular weights of the polymers synthesized from the lower-molecular-weight prepolymer (PCP9C) were lower with scCO2 compared to those of the polymers synthesized with N2. This apparently contradictory effect can be understood in terms of competition between rate-increasing effects of scCO2, such as greater chain mobility and a higher byproduct diffusion coefficient caused by plasticizing of the polymer by scCO2, and rate-diminishing effects associated with scCO2, such as lamellar thickening, higher crystallinity, and perhaps increased occlusion of end groups in crystalline regions.

Effect of prepolymer molecular weight on solid state polymerization of poly(bisphenol a carbonate) with nitrogen as a sweep fluid

AbstractThe effect of prepolymer molecular weight on the solid‐state polymerization (SSP) of poly(bisphenol A carbonate) was investigated using nitrogen (N2) as a sweep fluid. Prepolymers with different number–average molecular weights, 3800 and 2400 g/mol, were synthesized using melt transesterification. SSP of the two prepolymers then was carried out at reaction temperatures in the range 120–190 °C, with a prepolymer particle size in the range 20–45 μm and a N2 flow rate of 1600 mL/min. The glass transition temperature (Tg), number–average molecular weight (Mn), and percent crystallinity were measured at various times during each SSP. The phenyl‐to‐phenolic end‐group ratio of the prepolymers and the solid‐state synthesized polymers was determined using 125.76 MHz 13C and 500.13 MHz 1H nuclear magnetic resonance (NMR) spectroscopy. At each reaction temperature, SSP of the higher‐molecular‐weight prepolymer (Mn = 3800 g/mol) always resulted in higher‐molecular‐weight polymers, compared with the polymers synthesized using the lower molecular weight prepolymer (Mn = 2400 g/mol). Both the crystallinity and the lamellar thickness of the polymers synthesized from the lower‐molecular‐weight prepolymer were significantly higher than for those synthesized from the higher‐molecular‐weight prepolymer. Higher crystallinity and lamellar thickness may lower the reaction rate by reducing chain‐end mobility, effectively reducing the rate constant for the reaction of end groups. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4959–4969, 2008

Synthesis and characterization of biodegradable lactic acid‐based polymers by chain extension

AbstractBACKGROUND: Poly(lactic acid) (PLA), coming from renewable resources, can be used to solve environmental problems. However, PLA has to have a relatively high molecular weight in order to have acceptable mechanical properties as required in many applications. Chain‐extension reaction is an effective method to raise the molecular weight of PLA.RESULTS: A high molecular weight biodegradable lactic acid polymer was successfully synthesized in two steps. First, the lactic acid monomer was oligomerized to low molecular weight hydroxyl‐terminated prepolymer; the molecular weight was then increased by chain extension using 1,6‐hexamethylene diisocyanate as the chain extender. The polymer was characterized using 1H NMR analysis, gel permeation chromatography, differential scanning calorimetry and Fourier transform infrared spectroscopy. The results showed that the obtained polymer had a Mn of 27 500 g mol−1 and a Mw of 116 900 g mol−1 after 40 min of chain extension at 180 °C. The glass transition temperature (Tg) of the low molecular weight prepolymer was 47.8 °C. After chain extension, Tg increased to 53.2 °C. The mechanical and rheological properties of the obtained polymer were also investigated.CONCLUSION: The results suggest that high molecular weight PLA can be achieved by chain extension to meet conventional uses. Copyright © 2008 Society of Chemical Industry

Influence of molecular weight and free NCO content on the rheological properties of lithium lubricating greases modified with NCO-terminated prepolymers

In this work, the use of reactive diisocyanate-terminated polymeric materials as rheology modifiers of lubricating greases has been studied. Particularly, the influences that free NCO content, molecular weight and functionality of the reactive prepolymers exert on the rheological response and microstructure of lubricating greases were analyzed. With this aim, NCO-terminated prepolymers were prepared from several di and trifunctional polyols and polymeric MDI. Afterwards, the reaction between terminal isocyanate groups and the hydroxy group located in the hydrocarbon chain of the 12-hydroxystearate lithium soap, used as thickener, was promoted during processing of lubricating greases. Polymeric materials used as additives and final lubricating greases were characterized by FTIR, DSC and GPC techniques. The effectiveness of these reactive additives was tested by performing small-amplitude oscillatory shear (SAOS), as well as standardized mechanical stability tests, on final greases. The rheological response was related to the microstructure of these greases, characterized by means of atomic force microscopy (AFM). From the experimental results obtained, it may be concluded that the effectiveness of these polymeric additives to modify the rheology of greases is due to the progress of the reaction between terminal isocyanate groups and the hydroxy group of lithium soap. However, a large dependence on both free NCO content and prepolymer molecular weight was found. Experimental results confirm that a balance between prepolymer molecular weight and NCO content is necessary to reach an optimal rheological modification of lithium greases. Moreover, this balance is a function of grease ageing, due to the progress of the reaction promoted.

Thermomechanical properties of stereoblock poly(lactic acid)s with different PLLA/PDLA block compositions

Stereoblock poly(lactic acids) (sb-PLAs), having abundant enantiomeric compositions of poly(l-lactic acid) (PLLA), were successfully synthesized by solid-state polycondensation (SSP) of the melt blends of medium molecular weight prepolymers: both PLLA and poly(d-lactic acid) (PDLA) that had primarily been prepared by melt-polycondensation. The molecular weight of the resultant PLLA-rich sb-PLA was effectively enhanced by the dehydrative coupling of abundant PLLA molecules followed by the increase in homo-chiral crystallinity. These sb-PLA were successfully fabricated into polymer films by solution casting to analyze their crystalline morphology and properties. Both DSC and WAXS revealed preferential stereocomplexation in spite of concomitant homo-chiral crystallization for these sb-PLA. Particularly, exclusive sc crystallization was observed with melt-quenched sb-PLA represented whose PDLA ratio was above 15%. Therefore, the stereoblock structure can suppress the homo-chiral crystallization and increase the sc crystallinity even with the non-equivalent PLLA/PDLA compositions. Their thermal resistivity up to 200°C was supported by the dynamic mechanical analysis of the sb-PLA films.

Synthesis and Hydrolysis Behaviour of Poly(ester anhydrides) from Polylactone Precursors Containing Alkenyl Moieties

Hydroxyl-group functional polylactones were prepared and converted to acid- terminated polyesters in a reaction with a series of alkenylsuccinic anhydrides containing 8, 12, or 18 carbons in their alkenyl chains. These polyester precursors were then linked into higher molecular weight poly(ester anhydrides) containing alkenyl moieties in their polyester blocks. The hydrolysis behaviour of the poly(ester anhydrides) was found to depend on the thermal properties of the polyester precursors. For poly(ester anhydrides) prepared from low molecular weight prepolymers with thermal transitions below 37 degrees C, the presence of hydrophobic alkenyl chains in the polyester precursors slowed the rate of weight loss. Poly(ester anhydrides) prepared from higher molecular weight prepolymers showed the opposite weight-loss behaviour; i.e., the crystallinity and thermal transitions of the alkenyl chain-containing poly(ester anhydrides) were low, and the weight loss was faster than for poly(ester anhydrides) without the alkenyl chains. The differences in length of the alkenyl chain, as such, had little effect on the hydrolysis behaviour and thermal properties of the poly(ester anhydrides).

TEMPERATURE DEPENDENCE OF HARDNESS OF WEAKLY CROSS-LINKED POLY(METHYL METHACRYLATE) NETWORK FORMED BY LATE ADDITION OF THE CROSS-LINKER

ABSTRACT The size distribution of three-dimensional loops in a network structure influences the hardness of polymers. A narrow size distribution results in larger hardness than a broad distribution of the same average size. The loop size distribution can be changed by late addition of the cross-linker. In this work the change of hardness with temperature of late and weakly cross-linked poly(methyl methacrylate) was studied experimentally. The hardness of weakly and late cross-linked samples with different cross-linker mole fractions was determined at different temperatures changing from –15 to 170°C. In all temperature ranges, as the time of the addition of the cross-linker to the polymerizing reaction increases, first the hardness increases, passes through a maximum, and then decreases. The dependence of hardness on temperature was investigated by plotting log H (hardness) versus T (temperature) graphs. Two different behaviors were observed; one below and the other above the glass transition temperature. Log H versus T graphs resulted in two different regions with two different slopes. The slopes of the straight lines first decrease up to the prepolymer molecular weight corresponding to maximum hardness and then increase gradually until the highest prepolymer molecular weight.This situation shows that the network loop size distribution gets narrower up to the maximum hardness and then broadens gradually at very late additions of the cross-linker.