Robert L. Sammler

Correlation between glass transition temperature and chain structure for randomly crosslinked high polymers

The empirical form for the dependence, Tg(n) ≅ Tg(∞)·(1 + α/n), of the glass transition temperature Tg on the average number n of repeat units between crosslinks, is generalized for randomly crosslinked high polymers. The new form, Tg(n) ≅ Tg(∞) · [1 + c/(n·Nrot)], is based on a correlation study of data for 77 samples of 10 different sets of resins. The fitting parameter α is resolved into composition-dependent Nrot and composition-independent c terms. Nrot summarizes the average number of rotational degrees of freedom per repeat unit, and is estimated in a straightforward manner from the structure and mol fraction of each repeat unit. The value of c is found from data analysis to be 5 ± 2. The results of this work are consistent with expectations based on the entropy theory of glasses, and provide improved understanding and predictive ability for the properties of crosslinked polymers.

Structure and properties of aqueous methylcellulose gels by small-angle neutron scattering.

Cold, semidilute, aqueous solutions of methylcellulose (MC) are known to undergo thermoreversible gelation when warmed. This study focuses on two MC materials with much different gelation performance (gel temperature and hot gel modulus) even though they have similar metrics of their coarse-grained chemical structure (degree-of-methylether substitution and molecular weight distribution). Small-angle neutron scattering (SANS) experiments were conducted to probe the structure of the aqueous MC materials at pre- and postgel temperatures. One material (MC1, higher gel temperature) exhibited a single almost temperature-insensitive gel characteristic length scale (ζ(c) = 1090 ± 50 Å) at postgelation temperatures. This length scale is thought to be the gel blob size between network junctions. It also coincides with the length scale between entanglement sites measured with rheology studies at pregel temperatures. The other material (MC2, lower gel temperature) exhibited two distinct length scales at all temperatures. The larger length scale decreased as temperature increased. Its value (ζ(c1) = 1046 ± 19 Å) at the lowest pregel temperature was indistinguishable from that measured for MC1, and reached a limiting value (ζ(c1) = 450 ± 19 Å) at high temperature. The smaller length scale (ζ(c2) = 120 to 240 Å) increased slightly as temperature increased, but remained on the order of the chain persistence length (130 Å) measured at pregel temperatures. The smaller blob size (ζ(c1)) of MC2 suggests a higher bond energy or a stiffer connectivity between network junctions. Moreover, the number density of these blobs, at the same reduced temperature with respect to the gel temperature, is orders of magnitude higher for the MC2 gels. Presumably, the smaller gel length scale and higher number density lead to higher hot gel modulus for the low gel temperature material.

Compatibility of high polymers probed by interfacial tension

Imbedded-fiber retraction (IFR) has been applied to study the compatibility of high polymers. IFR measures the interfacial tension between two immiscible high-viscosity thermoplastic resins in their molten states. Ten nonreactive blend pairs were studied. One blend component was a poly(styrene-co-acrylonitrile-co-fumaronitrile) terpolymer resin (S/AN/FN). The other component was one of a set of ten S/AN resins with an AN level between 0 and 51%. These high-molecular-weight resins were particularly challenging for IFR since they were nearly isorefractive, had high melt viscosities (103–105 Pa s), and could chemically age when molten. Interfacial tensions γ12 ranged from 0.00 to 5.5 dyn/cm at 200 °C. Miscible bends had γ12 = 0 and a single Tg.Immiscible blends had γ12 > 0 and two Tgs. Compatibility was quantitatively assessed from the monotonic rise in γ12 as compatibility decreases. The results demonstrate that IFR can rank the compatibility of high polymers. It is expected that IFR can also rank the compatibility of polymers with similar Tgs,and rank the ability of additives to enhance blend compatibility.

Size-exclusion chromatography of ultrahigh molecular weight methylcellulose ethers and hydroxypropyl methylcellulose ethers for reliable molecular weight distribution characterization.

Size-exclusion chromatography (SEC) coupled with multi-angle laser light scattering (MALLS) and differential refractive index (DRI) detectors was employed for determination of the molecular weight distributions (MWD) of methylcellulose ethers (MC) and hydroxypropyl methylcellulose ethers (HPMC) having weight-average molecular weights (Mw) ranging from 20 to more than 1,000kg/mol. In comparison to previous work involving right-angle light scattering (RALS) and a viscometer for MWD characterization of MC and HPMC, MALLS yields more reliable molecular weight for materials having weight-average molecular weights (Mw) exceeding about 300kg/mol. A non-ideal SEC separation was observed for cellulose ethers with Mw>800kg/mol, and was manifested by upward divergence of logM vs. elution volume (EV) at larger elution volume at typical SEC flow rate such as 1.0mL/min. As such, the number-average molecular weight (Mn) determined for the sample was erroneously large and polydispersity (Mw/Mn) was erroneously small. This non-ideality resulting in the late elution of high molecular weight chains could be due to the elongation of polymer chains when experimental conditions yield Deborah numbers (De) exceeding 0.5. Non-idealities were eliminated when sufficiently low flow rates were used. Thus, using carefully selected experimental conditions, SEC coupled with MALLS and DRI can provide reliable MWD characterization of MC and HPMC covering the entire ranges of compositions and molecular weights of commercial interest.

Rheology of Cellulose Ether Excipients Designed for Hot Melt Extrusion

A new family of cellulosic ether polymeric excipients has been recently engineered for fabrication of amorphous solid dispersions of active pharmaceutical ingredients via hot-melt extrusion (HME). These hydroxypropyl methyl cellulose excipients enable plasticizer-free melt processing at much lower temperatures (135-160 °C) due to their substantially reduced glass transition temperatures ( Tg = 98-110 °C). The novel amorphous cellulose ethers were found to be rheologically solidlike well above their glass transition ( Tg + 70 °C). We demonstrate that in the pharmaceutically relevant HME processing temperature range these polymers behave similarly to yield-stress fluids and flow only when the applied stress exceeds a critical stress value. This critical stress value (0.50 ± 0.05 MPa, 150 °C) is surprisingly high but is easily achieved under typical HME conditions. The origin of their yield-stress fluidlike behavior is hypothesized to arise from hydrogen bonds of the HPMC materials that act as physical cross-links and do not relax within the measured temperature and time window unless the applied stress exceeds the critical stress value. Support for this hypothesis arises from infrared spectroscopic estimates of the free and bound hydrogen bond levels at end-use temperatures.