Insight into Nanoscale Network of Spray-dried Polymeric Particles

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Biological and Medical Applications of Materials and Interfaces

Insight into Nanoscale Network of Spray-dried Polymeric Particles: Role of Polymer Molecular Conformation Feng Wan, Flemming H. Larsen, Heloisa N. Bordallo, Camilla Foged, Jukka Rantanen, and Mingshi Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12475 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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ACS Applied Materials & Interfaces

Insight into Nanoscale Network of Spray-dried Polymeric Particles: Role of Polymer Molecular Conformation

Feng Wan1, Flemming Hofmann Larsen2, Heloisa Nunes Bordallo3, Camilla Foged1, Jukka Rantanen1, Mingshi Yang1,4*

1

Department of Pharmacy, Faculty of Health and Medical Sciences, University of Copenhagen,

Universitetsparken 2, 2100 Copenhagen, Denmark; 2 Department of Food Science, Faculty of Science, University of Copenhagen, Rolighedsvej 26, 1958 Frederiksberg C, Denmark; 3 Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100, Copenhagen, Denmark; 4 Wuya College of Innovation, Shenyang Pharmaceutical University, Wenhua Road 103, 110016, China *

Corresponding author: [email protected]

1 ACS Paragon Plus Environment

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Abstract: Poly (DL-lactic-co-glycolic acid) (PLGA) microparticles represent a promising formulation approach for providing steady pharmacokinetic/pharmacodynamic profiles of therapeutic drugs for a long period. Understanding and controlling the supramolecular structure of PLGA microparticles at a molecular level is a prerequisite for the rational design of well-controlled, reproducible sustained-release profiles. Herein, we reveal the role of PLGA molecular conformation in particle formation and drug release. The nanoscale network of PLGA microparticles spray-dried using the solvents with distinct polarities was investigated by using nuclear magnetic resonance (NMR) and neutron scattering. By employing chemometric method, we further demonstrate the evolution of nanoscale networks in spray-dried PLGA microparticles upon the water absorption. Our results indicate that PLGA molecules form more chain entanglements during spray drying when using the solvents with low polarity, where PLGA molecule adopts a more flexible, extended conformation, resulting in that the more resistant network against the water absorption in the spray-dried PLGA microparticles. This work underlines the role of PLGA molecular conformation in controlling formation and evolution of nanoscale network of spraydried PLGA microparticles and will have important consequences in achieving the customized drug release from PLGA microparticles.

Keywords: Spray-dried PLGA microparticles, nanoscale network, solid-state-NMR, neutron scattering, principal component analysis


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Introduction Biodegradable, injectable polymeric depot formulations (e.g., poly (lactic-co-glycolic acid) (PLGA) microparticles) represent a potent approach to improve efficacy and safety for a number of drug molecules by providing steady pharmacokinetic/pharmacodynamic profiles of drugs for a long period 1. Spray drying becomes increasingly attractive to pharmaceutical industry for the production of PLGA microparticles-based formulations due to its inherent ability to precisely manipulate critical particle properties (e.g., size, shape, and density) and the simple, economical manufacturing process for aseptic products

2-4

. Prior to spray drying, materials need to be dissolved or suspended in solvent(s) to form a

feed. In the spray drying process, the solvent(s) that constitute a great proportion of the feed undergo evaporation, resulting in dry materials. Arising from this is a significant interest around the effect of solvent(s) on the quality attributes (e.g., particle size, density, and release behavior) of spray-dried PLGA microparticles. Our previous study demonstrated how the interplay between the solvent power and the drying rate influences the formation and the physicochemical properties of spray-dried celecoxib-loaded PLGA microparticles 5. However, understanding and controlling of the polymeric particle formation at a molecular level during spray drying has not been reached. In addition, the complexity of drug release from PLGA microparticles-based depot formulations makes it difficult to achieve a customized release profile, which is indeed one of the major hurdles for the development of PLGA microparticles-based depot products

6-7

. Previous studies toward understanding and controlling

of the drug release from PLGA microparticles mainly focus on characterizing spatial drug distribution 5, drug-polymer interactions 8, water absorption 9-10, and the porosity of the microparticles 11-12. However, the role of PLGA molecular conformation in particle formation and drug release has not been fully explored. Polymer molecular conformation is highly solvent-dependent, i.e. polymer molecules present



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an extended conformation in an optimal solvent, whereas adopt a more compact coil structure in a poor solvent

13

. In this context, polymer molecules form the distinct transient networks via the chain

entanglements in the feed solution according to the solvents

14

. Our previous study indicated the

potential impact of PLGA conformational structure in the feed solution on drug release kinetics from spray-dried PLGA microparticles

15

. However, information is still lacking regarding how the PLGA

molecular conformation influences the nanoscale network of spray-dried PLGA microparticles, and the evolution of nanoscale network upon the water absorption during the drug release.

To deepen this understanding, we aimed at probing the nanoscale polymer network of PLGA microparticles spray-dried from the solvents with different intrinsic properties (e.g. dielectric constants (ɛ), evaporation rate, etc.). The solvent systems applied in the study can essentially result in the distinct drying kinetics and rheological properties of the feed solutions, which are expected to influence the formation of polymer chain entanglements in the spray-dried PLGA microparticles. By using nuclear magnetic resonance (NMR) and neutron scattering, the polymer molecular mobility in the spray-dried PLGA microparticles, which is closely associated with the supramolecular structure, was investigated. To further reveal the role of PLGA molecular conformation in drug release, the evolution of nanoscale networks in spray-dried PLGA microparticles upon the water absorption was investigated by using NMR spectroscopy in association with chemometric approach. The work provides a new understanding on the effect of PLGA molecular conformation on the formation of supramolecular structure of spraydried PLGA microparticles, and their evolution in the drug release process.

Experimental Section 4 ACS Paragon Plus Environment

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Materials: Poly(DL-lactic-co-glycolic acid) (PLGA, Resomer® RG 503 H, acid terminated, 50:50, Mw = 24,000-38,000) was purchased from Evonik (Essen, DE). Acetone (CH3COCH3, 99.9% HPLC grade), methanol (CH3OH, 99.9% HPLC grade) and dichloromethane (CH2Cl2, 99.9% HPLC grade) were purchased from Sigma–Aldrich (Poole, UK).

Viscosity of the feed solutions: The viscosity of the feed solutions was determined by using an Ubbelohde viscometer (Cannon instruments, PA, USA) at 25 ºC in a water bath. The intrinsic viscosity ([η]) of PLGA in the different solvents was determined by extrapolating the values for the specific viscosity ηsp as a function of the polymer concentration, c, using the limit theorem presented in equation 1 16: = lim→

(1)

The overlap concentration (c*) for PLGA in the different solvents was calculated according to equation 2 17:

∗ =

(2)

The polymer coil radius, Rcoil, in the different solvent systems was calculated from [η] by using equation 3

18

:

= 3 ∙ ⁄10 ∙ ! /$

(3)

where Mw is molecular weight of the polymer and NAV is Avogadro’s number.



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Preparation of PLGA microparticles by spray drying: PLGA microparticles were prepared using a Büchi B-290 spray dryer (Büchi Labortechnik AG, Postfach, CH) equipped with an inert loop B-295 (Büchi Labortechnik AG). The feeds used in the study are PLGA organic solutions (PLGA dissolved in organic solvents, Table 1). All samples were prepared at identical drying conditions (inlet temperature: 45 °C; outlet temperature: 30 °C; drying air flow rate: 22.5 m3/h; atomizing air flow rate: 742 L/h; and feed flow rate: 3 ml/min).

Characterization of spray-dried PLGA microparticles: The residual moisture of spray-dried PLGA microparticles were measured by thermogravimetric analysis (TGA) using a TGA 7 (Perkin Elmer, Waltham, Massachusetts, USA) under a nitrogen purge of 20 ml/min. Samples (approximately 5–10 mg) were loaded onto an open platinum pan and heated from 20 to 120°C at a scan rate of 10°C/min. The particle morphology of the spray-dried particles was visualized by using a scanning electron microscope (Tabletop Microscope TM3030, Hitachi High-Tech, Tokyo, JP) at an accelerating voltage of 5 kV. The samples were transferred onto sticky carbon tape and mounted on metallic stubs, followed by sputter coating with a 5 nm thick layer of gold to make the surfaces conductive.

1

H NMR relaxometry experiments: 1H NMR T2 relaxation experiments were performed using a Maran

Ultra NMR spectrometer (Resonance Instruments Ltd, Oxon, UK) operating at 23.3 MHz for 1H. The decay of the spin-spin relaxation (T2 decay) was measured with the single-pulse (90°) excitation (SPE) and Hahn-echo pulse (HEPS) sequences. The key parameters used during the experiments are listed in Table S1. The magnetization decay curves were initially analyzed using multicomponent fitting models,


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however, all three samples demonstrated one component from each magnetization decay curves (the free induction decay measured with SPE and T2 decay measured with HEPS). The detailed description of the methods and data analysis has been published previously 19-20.

13

C MAS NMR experiments: All samples were analysed by

13

C cross-polarization (CP) magic angle

spinning (MAS) NMR spectroscopy using a Bruker Avance 400 (9.4 T) NMR spectrometer (Bruker, Massachusetts, US) operating at Larmor frequencies of 400.13 and 100.63 MHz for 1H and

13

C,

respectively. All CP/MAS experiments were recorded at 298-338 K using a double-tuned CP/MAS probe equipped for 4 mm rotors employing a spin-rate of 6700 Hz, rf-field strengths of 83 kHz for both 1

H and

13

C, a contact time

21

of 2.0 ms, a recycle delay of 8 s and 224 scans. In addition, a series of

spin-lock experiments 22 were performed at 298-338 K utilizing a 1H spin-lock period from 3.0 to 30.0 ms in 10 equidistant steps, while the subsequent time for CP was maintained at 2.0 ms. For both experiments, the acquisition time was 12.0 ms for dry samples and 49.8 ms for hydrated samples during which TPPM 1H decoupling

23

was applied. All spectra were referenced to the carbonyl

resonance of α-glycine at 176.5 ppm (external sample). The 1H T1ρ was determined by fitting the intensity of the resonances in the spin-lock experiments as a function of spinlock time 22, 24. To investigate the impact of the nanoscale PLGA networks resulting from the use of the different solvents on the initial release behavior, the spray-dried PLGA microparticles were hydrated by mixing dry samples with D2O with PLGA (w/w) contents of either 25 or 50 % in the NMR rotor. Then

13

C

CP/MAS NMR spectra were recorded every 30 (for PLGA content of 50 %) or 60 min (for PLGA content of 25%) for a period of 24 hours to monitor spectral evolution in the hydration process.



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Hereafter these data were collected in a data matrix and subjected to Principal Component Analysis (PCA) 25 to access if the hydration characteristics differed among the samples.

Neutron scattering: Neutron scattering spectra were recorded using the ToF inverted geometry crystal analyzer spectrometer IRIS34 located at the ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory, Oxfordshire, UK. The samples were confined in a flat plate aluminum container and sealed inside an aluminum foil packet to avoid corrosion of the sample holder. Following a 2 h quasi-elastic spectra data collection at 310 K, the samples were cooled slowly, and elastic scans were conducted during the cooling and the subsequent heating. Quasi-elastic spectra were also recorded at base T, 200 K, and 250 K, using 2 h measurements at each temperature. Data reduction was performed using the Mantid program with detector grouping mode as ‘‘All’’. Data was exported into Excel for further analysis and were normalized by mass sample. Observation of quasi-elastic broadening indicates that diffusional motions occur in the observation time window of the instrument. The same two programs were used for the analysis of the elastic fixed elastic window scans. For these scans, data was also normalized to the total scattered intensity at base T after normalizing the data by the respective mass of the samples. The mean square displacement ⟨u2⟩ was calculated to determine the effective variation of all hydrogen atoms around their equilibrium position using the equation 4 reported previously 26-27: ln [S (Q, T, ω≈0)] = −⟨u2⟩Q2/3

(4)

Results and discussion


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Molecular mobility of spray-dried PLGA microparticles investigated by using relaxometry and

13

1

H NMR

C magic-angle spinning (MAS) NMR spectroscopy. Three solvents, i.e.

methanol, acetone, and dichloromethane, with different dielectric constants (ɛ) were selected to vary the polarity of solvent systems, thereby manipulate the PLGA molecular behavior in the feed solution and the evaporation rate (Table 1). From the intrinsic viscosity ([η]) of PLGA in the different solvents, PLGA molecules in dichloromethane exhibit a more extended conformation than PLGA molecules in ACE and the binary system (acetone:methanol=69:31, molar ratio), therefore, form the more chain entanglements in the feed solution (Table S2). PLGA microparticles were prepared by using spray drying under the identical process. The gentle drying conditions, especially the relatively low outlet temperature, were chosen due to the low glass transition temperatures of PLGA to prevent heavy deposition of PLGA in the cyclone and improve the yield of PLGA microparticles. According to the thermogravimetric analysis (TGA), only around 0.5% (w/w) of the residual solvent content was present in all three types of spray-dried microparticles (Table S3), suggesting that this inlet temperature is sufficiently high to evaporate organic solvents due to the volatile nature of the organic solvents used in this study. In addition, the influence of the residual solvents in NMR experiments is negligible. The spherical spray-dried PLGA microparticles with diameter of about 2-10 µm were obtained, irrespective of which solvent system applied (Figure S1). The molecular mobility and heterogeneity of the spray-dried PLGA microparticles were first investigated by using 1H NMR relaxometry to determine the transverse magnetization relaxation time (T2)

19-20

. The relaxometry data for all three samples was modeled by two T2 relaxation components

(Figure 1), which can be assigned to a relatively rigid fraction with largely restricted chain mobility (~30 µs, T2, r) and a mobile fraction with relatively high chain mobility (50-100 µs, T2, m). It should be



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noted that the multicomponent relaxation does not imply the phase separation, but rather suggests the presence of the populations of hydrogen atoms’ having the different mobility. In addition, PLGA is a well-known amorphous glassy polymer with glass transition temperature of approximately 38-45°C (molecular weight-dependent). In the previous studies, the spray-dried PLGA microparticles with or without drug loadings were also characterized as amorphous glass

5, 28

. The relative amount of each

relaxation component represents the amount of these particular hydrogen atoms in the samples. The dependence of T2 relaxation time on organic solvent applied was clearly observed (Figure 1a). In addition, the relative amount of T2, r component in the spray-dried PLGA microparticles from the binary solvents system (acetone:methanol=69:31, molar ratio), acetone, and dichloromethane was approximately 50%, 70% and 90%, respectively (Figure 1b). The short T2 relaxation time of the relatively rigid fraction could be attributed to the intermolecular hydrogen bonds in the cross-linked polymers. In contrast, the long T2 relaxation time of mobile fraction may represent the populations of hydrogen atoms with relatively high mobility (i.e., not involved in the formation of intermolecular hydrogen bonds)

19

. Therefore, the increased amount of T2, r component in the spray-dried PLGA

microparticles from the nonpolar solvents (e.g., acetone and dichloromethane) suggests a larger fraction of cross-linked polymers (chain entanglements) formed in the spray drying process. This could be explained by that the extended conformational structure of PLGA molecules in the solvents with lower dielectric constants facilitates the formation of chain entanglements in the spray drying process. Interestingly, the value of T2,

m

increased and the relative abundance decreased with the organic

solvents as following: acetone:methanol=69:31(molar ratio) < acetone < dichloromethane, seemingly suggesting that non-cross-linked PLGA molecules present the more flexible conformational structure in the spray-dried PLGA microparticles prepared using the solvents with lower dielectric constants.


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To further explore the molecular dynamics of the polymeric network, the spin-lattice relaxation time of protons in the rotating frame (1H T1ρ) was determined using spinning (CP/MAS) solid state NMR experiments

29

. The

13

13

C cross-polarization magic-angle

C CP/MAS NMR spectrum of PLGA

displays five isotropic resonances at 169.9 ppm (COO (L)), 167.8 ppm (COO (G)), 69.7 ppm (CH (L)), 61.3 ppm (CH2 (G)), and 16.8 ppm (CH3 (L)) (Figure S2), which is consistent with the previous studies 30

. The site-specific 1H T1ρ values (Figure 2 and Table S4) showed that the longest T1ρ relaxation times

were obtained for the spray-dried PLGA microparticles prepared using dichloromethane, indicating a strong hydrogen bonding of the cross-linked PLGA in these particles, leading to restricted segmental motion of the polymer chains. The shortest 1H T1ρ values were observed for the spray-dried PLGA microparticles prepared using the binary solvent mixture (acetone:methanol=69:31, molar ratio). The observations are in agreement with the 1H NMR relaxometry results. Furthermore, an increase in temperature resulted in smaller values of 1H T1ρ (Figure 2c and d) , which could be due to the backbone motions that will affect the efficiency of the polarization transfer from 1H to 13C. However, it should be noted that the 1H T1ρ values of the spray-dried PLGA microparticles prepared using solvents with lower dielectric constants (e.g. acetone and dichloromethane) at 308K were still higher than those of the spray-dried PLGA microparticles prepared by the binary solvent mixture (acetone:methanol=69:31, molar ratio). This indicates the differences in the network among the spray-dried PLGA microparticles from different solvents can be retained at physiological temperature (37 °C).

Molecular mobility of spray-dried PLGA microparticles investigated by using elastic incoherent neutron scattering (EINS). It should be noted that the above study of 1H T1ρ reflects the larger-scale cooperative chain motions (α-relaxation). Therefore, the intramolecular degrees of freedom (β11 ACS Paragon Plus Environment


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relaxation or secondary relaxations) were further investigated by using elastic incoherent neutron scattering (EINS) due to its ability to detect the motion of protons on very short time scale (pico-second) 31

. A linear decrease in the elastic intensity, because of thermal activation of the motions, was observed

with an increase in temperature from 2K to 160 K, followed by a pronounced drop in elastic intensity at approximately 160 K for the spray-dried PLGA microparticles prepared using acetone and dichloromethane, and 180 K for the spray-dried PLGA microparticles prepared using the binary solvent system (acetone:methanol=69:31) (Figure 3). The anomalous decrease in the elastic intensity is related to the onset of some type of diffusive motion, which manifests as QE scattering (Figure S3). Correspondingly, the mean square displacements (MSDs, ⟨u2⟩) of hydrogen atoms calculated from the elastic intensity showed an anharmonic increase in ⟨u2⟩ at around 160 K for the PLGA microparticles prepared by using acetone and dichloromethane, and 180 K for the PLGA microparticles prepared by using the binary solvent system (acetone:methanol=69:31) (Figure S4). As the transition happens at the temperatures below the glass transition temperature of PLGA (38-45 °C), it is likely related to the secondary relaxation (β-relaxation) that is a mostly local, intramolecular process

32-33

. These

observations, therefore, suggest that PLGA molecules exhibit a more rigid conformational structure in the

spray-dried

PLGA

microparticles

prepared

using

the

more

polar

solvent

system

(acetone:methanol=69:31, molar ratio); whereas a more flexible conformational structure in the spraydried PLGA microparticles prepared by using the nonpolar solvents (i.e., acetone and dichloromethane). It is reasonable because the PLGA molecules adopt a more flexible conformation in the solvent with lower dielectric constants; whereas present a rigid conformation in the solvents with higher dielectric constants due to strong PLGA intramolecular interaction


13

. The observations also

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imply that the conformational structures of PLGA molecules in feed solutions are retained in the solid state after spray drying.

Principal component analysis (PCA) of

13

C CP/MAS NMR spectral evolution of the spray-dried

PLGA microparticles upon water absorption. To further explore the impact of the distinct PLGA molecular arrangements in spray-dried microparticles on the initial release behavior, we investigated 13

C CP/MAS NMR spectral evolution of the spray-dried PLGA microparticles upon incubated with

water. The spray-dried PLGA microparticles were dispersed in water with 25% (w/w) and 50% (w/w) PLGA, respectively. The spectra were taken every 30 min for 24 h and analyzed by explorative data analysis approach using principal component analysis (PCA). The first two principal components (PCs) described 91.4% of the total variance in the

13

C CP/MAS NMR spectra (Figure 4a). The loading

vectors for PC1 are related to all five resonances at 169.9, 167.8, 69.7, 61.3, and 16.8 ppm having positive intensities (i.e., describing the PLGA content), whereas the loading vector for PC2 is characterized by negative intensities at 69.7, 61.3, and 16.8 ppm and positive intensities at 169.6 and 167.8 ppm (i.e., describing the difference between carbonyl and aliphatic groups) (Figure 4b). PC1 (88.1%) separates the samples incubated with 25% (w/w) of PLGA from the samples incubated with 50% (w/w) of PLGA in the score plot (Figure 4a), thus, clearly demonstrating the potential effect of microparticle:release media ratio on the release behavior

9-10

. The PC1 scores of the samples

incubated with 25% (w/w) of PLGA are lower than for the samples incubated with 50% (w/w) of PLGA, suggesting that an increase in the amount of water leads to the decreased network of spraydried PLGA microparticles within a very short time. PC2 (3.2%) describes the spectral evolution as a



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function of the incubation time. PC2 scores of the samples incubated with 50% (w/w) of PLGA progressively become lower as a function of the incubation time. At the first three time points this tendency is more pronounced for the spray-dried PLGA microparticles from the binary solvent system (ACE69) and the spray-dried PLGA microparticles from dichloromethane (DCM100) samples compared to the spray-dried PLGA microparticles from acetone (ACE100). When incubated with 25 % (w/w) PLGA only the first time points for ACE69 and DCM100 stand out from the rest. In this case, the PC2 score increased for ACE69 whereas it decreased for DCM100. The opposite effect was observed for the PC1 scores. The observed changes in scores reflect the polymer interaction with water. ACE100 incubated with both 25% and 50% (w/w) of PLGA, displaying the slowest shifting from positive to negative scores, seemingly presents a more resistant network against the water absorption (hydration) and retains the original network for longer time. In comparison, DCM100 is the most affected by hydration. Interestingly, ACE69 presents the ‘jumping’ from negative score to positive scores on both PC 1 and PC 2 at the initial time points (t1, t2). This seemingly suggests that water absorption results in the newly formed network at the initial time, which could be a result of rearrangement of PLGA molecules upon the water absorption. However, the rearrangement of PLGA molecules would lead to collapse of the macrostructure, thus contributing to the significant initial burst release 12.

Conclusion In conclusion, PLGA molecules form more chain entanglement in the spray-dried PLGA microparticles prepared using the solvents with low polarity than that prepared using the solvents with high polarity.


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This could be attributed to the fact that the extended conformational structure of PLGA chains in the solvents with low polarity facilitates the formation of chain entanglements during spray drying. In addition, explorative data analysis of

13

C MAS NMR spectral evolution of spray-dried PLGA

microparticles dispersed in water demonstrates that the spray-dried PLGA microparticles prepared using the polar solvent with low polarity (i.e. acetone) adopts a network which is more resistant against the water absorption. In comparison, the spray-dried PLGA microparticles prepared using the nonpolar solvent (i.e. dichloromethane) is most affected by hydration. Additionally, water absorption leads to the collapse of the macrostructure in the spray-dried PLGA microparticles prepared using the binary solvent system (acetone:methanol=69:31, molar ratio) at the initial time. The observations suggest that control of the molecular structure of PLGA and the nanoscale polymer network by tuning the polarity of the solvent system may represent an appealing way for achieving the customized drug release from PLGA microparticles.

Supporting Information. Table S1 Parameters used in 1H NMR T2 relaxation experiments; Table S2 Rheological characterization of the PLGA feed solutions used in the study; Table S3 Residual moisture of spray-dried PLGA microparticles; Table S4 Temperature dependence of spin lattice relaxation time in the rotating frame of protons (1H T1ρ, ms) in spray-dried PLGA microparticles prepared by using different solvents; Figure S1 Representative SEM images of spray-dried PLGA microparticles; Figure S2 13C CP/MAS NMR spectra of spray-dried PLGA microparticles prepared using acetone; Figure S3 Temperature evolution of the quasielastic broadening; Figure S4 Mean-square displacements (⟨u2⟩) derived from the elastic-scattered intensity according to a harmonic Debye model.



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Acknowledgement This work was funded by The Danish Council for Technology and Innovation via the Innovation Consortium NanoMorph (grant number: 952320/2009), The Drug Research Academy and The Danish Agency for Science, Technology and Innovation. FW acknowledges the financial support from The Danish Council for Independent Research, Technology and Production Sciences (grant number: DFF– 4093-00062). The authors are grateful to Dr. Vicky García Sakai and Dr. Mark Telling at the ISIS Neutron and Muon Facility (UK) for technical assistance and discussion of experiments performed using the IRIS backscattering spectrometer.

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Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Critical properties of solvents applied in the study Solvent system acetone:methanol= 69 : 31a

Dielectric

Solubility of

Evaporation rate

constant (ɛ)

PLGA

(n-butyl acetate = 1.0)

20.7~32.7 b

Poor

6.3~14.4 c

acetone

20.7

Good

14.4

dichloromethane

8.9

Good

25.0

p.s. a: the number is molar ratio; b: dielectric constant of methanol is 32.7; c: evaporation rate (n-butyl acetate = 1.0) of methanol is 6.3.



100

a

T2 Relaxation Time (µs)

T2,r

T2,m

80

60

40

20

0

ACE69

ACE100

DCM100

Sample ID 100

Phase Composition (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

80

b Rigid Fraction Mobile Fraction

60

40

20

0

ACE69

DCM100

ACE100

Sample ID Figure 1. Dependence of T2 relaxation time (A) and phase composition (B) on the organic solvent applied. In the term of Sample ID, ACE and DCM represent the solvent systems applied to prepare the spray-dried PLGA microparticles, e.g., ACE69, ACE100 and DCM100 represent that the solvent systems are acetone:methanol=69:31 (molar ratio), acetone, and dichloromethane, respectively.


Page 21 of 24

27

a

26

b

ACE69 ACE100 DCM100

16

21.0

308K

H T1ρρ (ms)

14

20.5

20.0

13

1

H T1ρρ (ms)

17

ACE69 ACE100 DCM100

25

1

298K

12

2

2

1

1

0

0

COO (L)/169.9 ppm

COO (G)/167.8 ppm

25

20

20

H T1ρρ (ms)

H T1ρρ (ms)

c

15

COO (G)/167.8 ppm

COO (L)/169.9 ppm

25

d

15

1

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

5

10

ACE69 ACE100 DCM100

5

COO (L) /169.9 ppm 0 290

300

ACE69 ACE100 DCM100

310

COO (G) /168.8 ppm 320

330

0 290

340

300

Temperature (K)

310

320

330

340

Temperature (K)

Figure 2. Comparison of the spin lattice relaxation time in the rotating frame of protons (1H T1ρ) in spray-dried PLGA microparticles prepared by using different solvents (a, at 298K, b, at 308K); the temperature dependence of 1H T1ρ of COO (L) (c) and COO (G) (d) in spray-dried PLGA microparticles prepared by using different solvents. In the term of Sample ID, ACE and DCM represent the solvent systems applied to prepare the spray-dried PLGA microparticles, e.g., ACE69, ACE100 and DCM100 represent that the solvent systems are acetone:methanol=69:31 (molar ratio), acetone, and dichloromethane, respectively.



1.00

180K

S (Q, T, ω≈0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

0.95

160K

0.90

0.85

0.80 0

50

100

150

200

250

300

350

Temperature (K)

Figure 3. Elastically scattered intensity integrated over the elastic line as a function of temperature. Black square: ACE69; red circle: ACE100; blue triangle: DCM100. In the term of Sample ID, ACE and DCM represent the solvent systems applied to prepare the spray-dried PLGA microparticles, e.g., ACE69, ACE100 and DCM100 represent that the solvent systems are acetone:methanol=69:31 (molar ratio), acetone, and dichloromethane, respectively. The arrows mark the temperatures at which an-harmonic onsets occur.


Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Principal component analysis of 13C MAS NMR spectra of spray-dried PLGA microparticles incubated with water (triangle: 25% (w/w) PLGA content; circles: 50% (w/w) PLGA content). a: score plot of PC 1 against PC 2 for all samples colored according to the organic solvents applied for the preparation, blue: ACE69; red: ACE100; black: DCM100. b: loading vectors of PC 1 and PC 2. In the term of Sample ID, ACE and DCM represent the solvent systems applied to prepare the spray-dried PLGA microparticles, e.g., ACE69, ACE100 and DCM100 represent that the solvent systems are acetone:methanol=69:31 (molar ratio), acetone, and dichloromethane, respectively.



Table of Contents/Abstract graphic:


Page 24 of 24

Insight into Nanoscale Network of Spray-dried Polymeric Particles

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