Determination of Chain Flip Rates in Poly(ethylene) Crystallites by Solid-State Low-Field 1H NMR for Two Different Sample Morphologies

Characterization of Controlled Release Starch-Nimodipine Implant for Antispasmodic and Neuroprotective Therapies in the Brain

Parenteral depot systems can provide a constant release of drugs over a few days to months. Most of the parenteral depot products on the market are based on poly(lactic acid) and poly(lactide-co-glycolide) (PLGA). Studies have shown that acidic monomers of these polymers can lead to nonlinear release profiles or even drug inactivation before release. Therefore, finding alternatives for these polymers is of great importance. Our previous study showed the potential of starch as a natural and biodegradable polymer to form a controlled release system. Subarachnoid hemorrhage (SAH) is a life-threatening type of stroke and a major cause of death and disability in patients. Nimotop® (nimodipine (NMD)) is an FDA-approved drug for treating SAH-induced vasospasms. In addition, NMD has, in contrast to other Ca antagonists, unique neuroprotective effects. The oral administration of NMD is linked to variable absorption and systemic side effects. Therefore, the development of a local parenteral depot formulation is desirable. To avoid the formation of an acidic microenvironment and autocatalytic polymer degradation, we avoided PLGA as a matrix and investigated starch as an alternative. Implants with drug loads of 20 and 40% NMD were prepared by hot melt extrusion (HME) and sterilized with an electron beam. The effects of HME and electron beam on NMD and starch were evaluated with NMR, IR, and Raman spectroscopy. The release profile of NMD from the systems was assessed by high-performance liquid chromatography. Different spectroscopy methods confirmed the stability of NMD during the sterilization process. The homogeneity of the produced system was proven by Raman spectroscopy and scanning electron microscopy images. In vitro release studies demonstrated the sustained release of NMD over more than 3 months from both NMD systems. In summary, homogeneous nimodipine-starch implants were produced and characterized, which can be used for therapeutic purposes in the brain.

Chain Dynamics of Partially Disentangled UHMWPE around Melting Point Characterized by 1H Low-Field Solid-State NMR

Melts of ultrahigh molecular weight polyethylene (UHMWPE) entangled significantly, suffering processing difficulty. In this work, we prepared partially disentangled UHMWPE by freeze-extracting, exploring the corresponding enchantment of chain mobility. Fully refocused ¹H free induction decay (FID) was used to capture the difference in chain segmental mobility during the melting of UHMWPE with different degrees of entanglement by low-field solid-state NMR. The longer the polyethylene (PE) chain is in a less-entangled state, the harder the process of merging into mobile parts after detaching from crystalline lamella during melting. ¹H double quantum (DQ) NMR was further used to obtain information caused by residual dipolar interaction. Before melting, the DQ peak appeared earlier in intramolecular-nucleated PE than in intermolecular-nucleated PE because of the strong constraints of crystals in the former one. During melting, less-entangled UHMWPE could keep disentangled while less-entangled high density polyethylene (HDPE) could not. Unfortunately, no noticeable difference was found in DQ experiments between PE melts with different degrees of entanglement after melting. It was ascribed to the small contribution of entanglements compared with total residual dipolar interaction in melts. Overall, less-entangled UHMWPE could reserve its disentangled state around the melting point long enough to achieve a better way of processing.

Evidence for complexation-induced micro-extension of poly(vinyl alcohol) chains in interphase and amorphous domains from solid-state NMR

The three-phase structure of poly(vinyl alcohol) (PVA)-iodine complexes was elucidated by solid-state NMR (SSNMR), of which the micro-extension of the PVA segment in the interphase and amorphous domains was directly confirmed. The three-phase structure of the PVA-iodine complex was decomposed by the inverse ¹³C T₁-filter, where ¹³C NMR resonance lines of a C(H) triplet were observed in all three phases. The chain axis of ∼26% extended chains in the interphase deviates 35°-50° relative to the stretching direction, while there is only a 1° deviation for the extended chains in the crystalline domain. The increasing iodine concentration results in the increment of hydrogen-bonded C(H) fractions in both the amorphous and interphase domains, while the distribution of different C(H) fractions remains almost constant in the crystalline domain. Such an increment results from the locally extended PVA chains induced by polyiodine species (I₃^-/I₅^-), since the hydrogen bond(s) (HBs) required a specific direction. Direct evidence for this comes from the similar ¹³C CP/MAS spectra of C(H) in the three phases between unoriented iodine-doped PVA and highly oriented pure PVA. This supports the aggregation model for the formation mechanism of PVA-iodine complexes, where the PVA chain takes an extended zig-zag conformation.

An in situ stretching instrument combined with low field nuclear magnetic resonance (NMR): Rheo-Spin NMR

An in situ stretching instrument combined with low field nuclear magnetic resonance (LF-NMR) was designed and developed, namely, Rheo-Spin NMR. The time resolved stress-strain curve together with the corresponding NMR signal can be simultaneously obtained. The Rheo-Spin NMR contains the functional modules, including (1) the in situ stretching module, (2) the NMR signal acquisition module, and (3) the cavity of the NMR positioning module. The unique ring-like shape of the sample is used to replace the traditional dumbbell sample due to limited space in the NMR probe, and the whole ring-like sample will be deformed during the uniaxial stretching process, which avoids the generation of interference signals from the undeformed sample. The designed stretching assembly made by zirconia ceramics is manufactured to match and stretch the ring-like samples. The strain rate can be tuned within the range of 10^-5-10^-2 s^-1 with the maximum stretching ratio λ_max of ∼3.8. The in situ stretching experiments combined with LF-NMR were carried out successfully with natural rubber of different fractions of carbon black. The time-resolved T₂ relaxometry was adopted to evaluate segmental relaxation during uniaxial deformation which, for the first time, provides the direct and in situ molecular dynamics information. The Rheo-Spin NMR is promising to provide more in-depth insights into the structure and dynamics evolution of polymer products under real service conditions.

Network structure of swollen iodine-doped poly(vinyl alcohol) amorphous domain as characterized by low field NMR

The network structure in the amorphous domain of swollen iodine-doped poly(vinyl alcohol) (PVA) was systematically investigated by low-field (LF) NMR techniques to reveal the PVA-iodine complex formation mechanism. Three PVA-iodine complexes were obtained under different iodine concentrations (c_iodine) of KI/I₂ solution: (i) c_iodine < 0.1 M: PVA-I₃^-/I₅^- complex only exists in the non-crystalline region, (ii) 0.1 M < c_iodine < 1 M: formation of PVA-I₃^- complex I, and (iii) c_iodine > 1 M: formation of PVA-I₃^- complex II. It was found that there is no intermediate-magnitude chain motion of PVA under dyeing conditions to induce the substance exchange, as evidenced by the unchanged second moment M₂ (∼1.2 × 10⁴ m s^-2) at elevated temperature (<380 K). The introduction of iodine ions can affect the chain mobility of the interphase and mobile regions. With increasing c_iodine, the chain dynamics become more restricted, as detected by the faster decay of the T₂ relaxometry results, which further accelerates the complexation process. The residual dipolar coupling strength, D_res, obtained by the more quantitative double-quantum (DQ) NMR, increases abruptly at c_iodine > 1 M. This suggests more constraints form in the amorphous network for the PVA-I₃^- complex II system. The constant defects fraction further reveals that the complexation prefers to happen along the tie chains. These results supply a possible formation pathway for the PVA-iodine complexes.

Chain dynamics and crystalline network structure of poly[R-3-hydroxybutyrate-co-4-hydroxybutyrate] as revealed by solid-state NMR

The chain dynamics and crystalline network structure of poly[R-3-hydroxybutyrate-co-4-hydroxybutyrate] (P(3HB-co-4HB)) were systematically investigated by the combination of various solid-state NMR techniques. High-resolution 13C cross-polarization (CP) and direct-polarization (DP) MAS with selective recycle delay times were first used to check the presence or absence of the 4HB unit in the crystalline domain. The results show that the 4HB unit is excluded from the crystalline domain. Afterward, 1H MAS Nuclear Overhauser Effect Spectroscopy (NOESY) with different mixing times was used, which shows that no micro-phase separation exists in the amorphous domain. 1H magic-sandwich-echo (MSE)-FID at elevated temperature shows the absence of motions on a timescale of 100 μs and below in the crystalline domain, as evidenced by the invariant second moment M2 of the proton line shape. Finally, the crystalline based network density was characterized directly by magic and polarization echo (MAPE)-double quantum (DQ) NMR, which shows a significant decreasing tendency after 80 °C. Such a decreasing crystalline network density, together with the reduced relaxation time, results in the significant decrement of the maximum stretch ratio and modulus in the high-temperature region.

Crystallinity of polyolefins with large side groups by low-field 1H NMR T2 relaxometry: Isotactic Polybutene-1 with form II and I crystals

Phase composition and molecular mobility were studied using ¹H NMR T₂ relaxometry in isotactic polybutene-1 (iPB-1) with two polymorphs - form I and II crystals. Several types of NMR relaxation methods and data analysis were evaluated for determining the most reliable way for studying physical phases in iPB-1. Three-phase model provided the most appropriate description of the phase composition in iPB-1, i.e., a crystal-amorphous interface separates the crystalline and the amorphous phases. Due to complex molecular mobility in iPB-1, the amount of rigid fraction should be considered as NMR crystallinity number. Two types of chain segments are present in the amorphous phase: (1) chain segments with anisotropic mobility due to chain anchoring to crystals and chain entanglements; and (2) highly mobile chain end segments. The polymorphic phase II to I transition causes significant immobilization of polymer chains in the crystalline and the amorphous phases. Molecular weight of iPB-1 largely influences phase composition and molecular mobility in crystalline and amorphous phases.

Single-shot measurement of solids and liquids T1 values by a small-angle flip-flop pulse sequence

We propose the small-angle flip-flop (SAFF) pulse sequence as an alternative procedure for the rapid measurement of the ¹ H spin-lattice relaxation time in the laboratory frame (T₁ ) of solid and liquid substances, in a time-domain NMR experiment. Based on the original flip-flop pulse sequence, this technique allows the fast estimation of T₁ values of samples that require minutes to hours of acquisition time if traditional pulse sequences are employed. We have applied SAFF to different substances, with T₁ ranging from microseconds up to seconds, including natural clays, polymers, and organic and inorganic solvents. We also demonstrate the potential of the pulse sequence in the real-time monitoring of dynamic processes, such as the conformational changes of polymeric materials during heating. The results we obtained with SAFF are comparable with those acquired with the inversion-recovery pulse sequence, with the addition of several benefits. This pulse sequence obeys steady-state and magnetization-conserving principles, making it possible to dismiss the need for relaxation delay times of the order of 5T₁ . SAFF has shown high sensitivity in the resolution of individual components of T₁ in multiexponential systems and can be easily integrated to well-established pulse sequences, such as Magic Sandwich Echo and Carr-Purcell-Meiboom-Gill, for the single-shot determination of T₁ and T₂ or T_2* .

Data on tensile tests and NMR measurements of poly(vinilidene fluoride) before and after stress relaxation

Poly(vinilidene fluoride) was characterized before and after stress relaxation by tensile tests and time-domain nuclear magnetic resonance (TD-NMR). Tensile tests were performed to provide mechanical properties, focused on the data of elastic modulus for this matter. The TD-NMR technique was used to calculate the fraction of crystalline, constrained amorphous and free amorphous phase, and the transversal relaxation time of each of these phases.

Dipolar filtered magic-sandwich-echoes as a tool for probing molecular motions using time domain NMR

We present a simple ¹H NMR approach for characterizing intermediate to fast regime molecular motions using ¹H time-domain NMR at low magnetic field. The method is based on a Goldmann Shen dipolar filter (DF) followed by a Mixed Magic Sandwich Echo (MSE). The dipolar filter suppresses the signals arising from molecular segments presenting sub kHz mobility, so only signals from mobile segments are detected. Thus, the temperature dependence of the signal intensities directly evidences the onset of molecular motions with rates higher than kHz. The DF-MSE signal intensity is described by an analytical function based on the Anderson Weiss theory, from where parameters related to the molecular motion (e.g. correlation times and activation energy) can be estimated when performing experiments as function of the temperature. Furthermore, we propose the use of the Tikhonov regularization for estimating the width of the distribution of correlation times.

Molecular Dynamics in the Crystalline Regions of Poly(ethylene oxide) Containing a Well-Defined Point Defect in the Middle of the Polymer Chain

The chain mobility in crystals of a homopolymer of poly(ethylene oxide) (PEO) with 22 monomer units (PEO₂₂) is compared with that of a PEO having the identical number of monomer units but additionally a 1,4-disubstituted 1,2,3-triazole (TR) point defect in the middle of the chain (PEO₁₁-TR-PEO₁₁). In crystals of PEO₂₂, the characteristic α_c-relaxation (helix jumps) is detected and the activation energy of this process is calculated from the pure crystalline ¹H FIDs to 67 kJ/mol. PEO₁₁-TR-PEO₁₁ exhibits a more complex behavior, i.e. a transition into the high temperature phase HTPh is noticed during heating in the temperature range between -5 and 10 °C which is attributed to the incorporation of the TR ring into the crystalline lamellae. The crystal mobility of the low temperature phase LTPh of PEO₁₁-TR-PEO₁₁ is in good agreement with PEO₂₂ since helical jump motions could also be detected by analysis of the ¹H FIDs and the corresponding values of their second moments M₂. In contrast, the high temperature phase of PEO₁₁-TR-PEO₁₁ shows a completely different behavior of the crystal mobility. The crystalline PEO chains are rigid in this HTPh on the time scale of both, the ¹H time-domain technique and in ¹³C MAS CODEX NMR spectroscopy, i.e. the α_c-mobility of PEO in the HTPh of PEO₁₁-TR-PEO₁₁ is completely suppressed and the PEO₁₁ chains are converted into a crystal-fixed polymer due to the incorporation of the TR rings into the crystal structure. However, the TR defect of PEO₁₁-TR-PEO₁₁ shows in the HTPh characteristic π-flip motions with an Arrhenius type activation energy of 223 kJ/mol measured by dielectric relaxation spectroscopy. This motion cannot be observed by corresponding ¹³C MAS CODEX NMR measurements due to an interfering spin-dynamic effect.

Basic principles of static proton low-resolution spin diffusion NMR in nanophase-separated materials with mobility contrast

We review basic principles of low-resolution proton NMR spin diffusion experiments, relying on mobility differences in nm-sized phases of inhomogeneous organic materials such as block-co- or semicrystalline polymers. They are of use for estimates of domain sizes and insights into nanometric dynamic inhomogeneities. Experimental procedures and limitations of mobility-based signal decomposition/filtering prior to spin diffusion are addressed on the example of as yet unpublished data on semicrystalline poly(ϵ-caprolactone), PCL. Specifically, we discuss technical aspects of the quantitative, dead-time free detection of rigid-domain signals by aid of the magic-sandwich echo (MSE), and magic-and-polarization-echo (MAPE) and double-quantum (DQ) magnetization filters to select rigid and mobile components, respectively. Such filters are of general use in reliable fitting approaches for phase composition determinations. Spin diffusion studies at low field using benchtop instruments are challenged by rather short (1)H T1 relaxation times, which calls for simulation-based analyses. Applying these, in combination with domain sizes as determined by small-angle X-ray scattering, we have determined spin diffusion coefficients D for PCL (0.34, 0.19 and 0.032nm(2)/ms for crystalline, interphase and amorphous parts, respectively). We further address thermal-history effects related to secondary crystallization. Finally, the state of knowledge concerning the connection between D values determined locally at the atomic level, using (13)C detection and CP- or REDOR-based "(1)H hole burning" procedures, and those obtained by calibration experiments, is summarized. Specifically, the non-trivial dependence of D on the magic-angle spinning (MAS) frequency, with a minimum under static and a local maximum under moderate-MAS conditions, is highlighted.