Effect of Low-Concentration Polymers on Crystal Growth in Molecular Glasses: A Controlling Role for Polymer Segmental Mobility Relative to Host Dynamics.

Low-concentration polymers can strongly influence crystal growth in small-molecule glasses, a phenomenon important for improving physical stability against crystallization. We measured the velocity of crystal growth in two molecular glasses, nifedipine (NIF) and o-terphenyl (OTP), each doped with four or five different polymers. For each polymer, the concentration was fixed at 1 wt % and a wide range of molecular weights was tested. We find that a polymer additive can strongly alter the rate of crystal growth, from a 10-fold reduction to a 10-fold increase. For a given polymer, increasing molecular weight slows down crystal growth and the effect saturates around DP = 100, where DP is the degree of polymerization. For all the systems studied, the polymer effect on crystal growth rate forms a master curve in the variable (Tg,polymer - Tg,host)/Tcryst, where Tg is the glass transition temperature and Tcryst is the crystallization temperature. These results support the view that a polymer's effect on crystal growth is controlled by its segmental mobility relative to the host-molecule dynamics. In the proposed model, crystal growth rejects impurities and creates local polymer-rich regions, which must be traversed by host molecules to sustain crystal growth at rates determined by polymer segmental mobility. Our results do not support the view that host-polymer hydrogen bonding plays a controlling role in crystal growth inhibition.

Tensile Fracture of Molecular Glasses Studied by Differential Scanning Calorimetry: Reduction of Heat Capacity by Lateral Constraint.

Molecular glasses indomethacin and ortho-terphenyl were formed and fractured by cooling a liquid on a less thermally expansive substrate. In-plane tension was created by the mismatch of thermal expansion coefficients and accumulated to cause catastrophic network fracture. Differential scanning calorimetry was used to characterize the process. The heat of fracture exceeds by 10 times the strain energy released, and matches the excess enthalpy stored by an elastic film that is cooled under lateral constraint. The constrained film has a smaller heat capacity than a free-standing film, by approximately 0.01 J/g/K or 1%. This allows the constrained film to reach higher enthalpy on cooling and the excess enthalpy is released at fracture.

Fast Crystal Growth in o-Terphenyl Glasses: A Possible Role for Fracture and Surface Mobility.

Molecular liquids can develop a fast mode of crystal growth ("GC growth") near the glass transition temperature. This phenomenon remains imperfectly understood with several explanations proposed. We report that GC growth in o-terphenyl conserves the overall volume, despite a 5% higher density of the crystal, and produces fine crystal grains with the same unit cell as normally grown crystals. These results indicate that GC growth continuously creates voids and free surfaces, possibly by fracture. This aspect of the phenomenon has not been considered by previous treatments and is a difficulty for those models that hypothesize a 5% strain without voids. Given the existence of even faster crystal growth on the free surface of molecular glasses, we consider the possibility that GC growth is facilitated by fracture and surface mobility. This notion has support from the fact that GC growth and surface growth are both highly correlated with surface diffusivity and with fast crystal growth along preformed cracks in the glass.

Fast Surface Crystal Growth on Molecular Glasses and Its Termination by the Onset of Fluidity.

Organic glasses can grow crystals much faster on the free surface than in the interior, a phenomenon important for fabricating stable amorphous materials. This surface process differs from and is faster than the glass-to-crystal (GC) growth mode existing in the bulk of molecular glasses. We report that similar to GC growth, surface crystal growth terminates if glasses are heated to gain fluidity. In their steady growth below the glass transition temperature Tg, surface crystals rise above the amorphous surface while spreading laterally and are surrounded by depressed grooves. Above Tg, the growth becomes slower, sometimes unstable. This damage is stronger on segregated needles (α indomethacin, nifedipine, and o-terphenyl) than on crystals growing in compact domains (γ indomethacin). This effect arises because the onset of liquid flow causes the wetting and embedding of upward-growing surface crystals. Segregated needles are at greater risk because their slow-growing flanks appear stationary relative to liquid flow at a low temperature. The disruption of surface crystal growth by fluidity supports the view that the process occurs by surface diffusion, not viscous flow. Compared to the bulk GC mode, surface crystal growth is disrupted less abruptly by fluidity. Nevertheless, to the extent that fluidity damages them, both processes are solid-state phenomena terminated in the liquid state.

Fast crystal growth from organic glasses: comparison of o-terphenyl with its structural analogs.

Crystal growth kinetics and liquid dynamics of 1,2-diphenylcyclopentene (DPCP) and 1,2-diphenylcyclohexene (DPCH) were characterized by optical microscopy and dielectric spectroscopy. These two molecules are structurally homologous and dynamically similar to the well-studied glassformer ortho-terphenyl (OTP). In the supercooled liquid states of DPCP and DPCH, the kinetic component of crystal growth ukin has a power law relationship with the primary structural relaxation time τα, ukin [proportionality] τα(­ξ) (ξ ≈ 0.7), similar to OTP and other fragile liquids. Near the glass transition temperature (Tg), both DPCP and DPCH develop much faster crystal growth via the so-called GC (glass to crystal) mode, again similar to the behavior of OTP. We find that the α-relaxation process apparently controls the onset of GC growth, with GC growth possible only at sufficiently low fluidity. These results support the view that GC crystal growth can only occur in systems where the liquid and crystal exhibit similar local packing arrangements.

Termination of Solid-State Crystal Growth in Molecular Glasses by Fluidity.

Fast crystal growth can abruptly emerge as molecular liquids are cooled to become glasses, a phenomenon presently unknown for other materials. This glass-to-crystal (GC) mode can cause crystallization rates orders of magnitude faster than those predicted by standard models. While GC growth is known for 12 systems, its features vary greatly with growth rates spanning a factor of 10(4). We report that the general condition for GC growth to exist is that liquid diffusion be slow relative to crystal growth according to D/u < 7 pm. This condition holds for all liquids exhibiting GC growth and suggests that the phenomenon is a solid-state process terminated by fluidity. GC growth must solidify several molecular layers before rearrangement by diffusion. We propose that GC growth propagates by a nonequilibrium crystal/liquid interface 3 nm wide that solidifies by local mobility. These results explain the prevalence of GC growth among organic liquids and guide its discovery in other materials.

Low-concentration polymers inhibit and accelerate crystal growth in organic glasses in correlation with segmental mobility.

Crystal growth in organic glasses has been studied in the presence of low-concentration polymers. Doping the organic glass nifedipine (NIF) with 1 wt % polymer has no measurable effect on the glass transition temperature Tg of host molecules, but substantially alters the rate of crystal growth, from a 10-fold reduction to a 30% increase at 12 °C below the host Tg. Among the polymers tested, all but polyethylene oxide (PEO) inhibit growth. The inhibitory effects greatly diminish in the liquid state (at Tg + 38 °C), but PEO persists to speed crystal growth. The crystal growth rate varies exponentially with polymer concentration, in analogy with the polymer effect on solvent mobility, though the effect on crystal growth can be much stronger. The ability to inhibit crystal growth is not well ordered by the strength of host-polymer hydrogen bonds, but correlates remarkably well with the neat polymer's Tg, suggesting that the mobility of polymer chains is an important factor in inhibiting crystal growth in organic glasses. The polymer dopants also affect crystal growth at the free surface of NIF glasses, but the effect is attenuated according to the power law us ∝ ub(0.35), where us and ub are the surface and bulk growth rates.

Fast Crystal Growth Induces Mobility and Tension in Supercooled o-Terphenyl.

A photobleaching method was used to measure the reorientation of dilute probes in liquid o-terphenyl near a crystal growth front. Near the glass-transition temperature Tg, mobility in the supercooled liquid was enhanced within ∼10 µm of the crystal growth front, by as much as a factor of 4. This enhanced mobility appears to be caused by tension created in the sample as a result of the density difference between the supercooled liquid and crystal. The maximum observed mobility enhancement corresponds to a tension of about -8 MPa, close to the cavitation limit for liquid o-terphenyl. Whereas the observed mobility near the growing crystal is not large enough to explain the extraordinary fast crystal growth observed near Tg in o-terphenyl and some other low-molecular-weight glassformers, these observations suggest that cavitation or fracture plays a key role in releasing tension and allowing fast crystal growth to occur at a steady rate.