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Transient Fiber Mats of Electrospun Poly(Propylene Carbonate) Composites with Remarkable Mechanical Strength Peter Ohlendorf, Alexander Ruyack, Amanda K. Leonardi, Chengjian Shi, Christine Cuppoletti, Ian Bruce, Amit Lal, and Christopher K. Ober ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017

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ACS Applied Materials & Interfaces

Transient Fiber Mats of Electrospun Poly(Propylene Carbonate) Composites with Remarkable Mechanical Strength Peter Ohlendorf, † Alexander Ruyack, ‡ Amanda Leonardi, † Chengjian Shi, † Christine Cuppoletti,§ Ian Bruce,§ Amit Lal, ‡ and Christopher K. Ober*, †

Department of Material Science and Engineering, Cornell University, Ithaca, NY 14853, USA ‡

Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USA §

SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025-2000, USA

KEYWORDS:

transient materials; electrospun poly(propylene carbonate); nanocomposite;

tunable mechanical properties; triggered decomposition; UV-degradable; thermally degradable; OMMT enhanced fiber mats

ABSTRACT: Polymers with a triggered decomposition are attractive for an array of applications ranging from patterning to transient packaging materials as well as for environmental protection. This work showed for the first time UV and thermally triggered transience in fiber mats using poly(propylene carbonate) (PPC) composites. The electrospun

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PPC-composite fiber mats combine excellent decomposition performance (due to the high surface to volume ratio) with high stiffness and thus represent a new class of materials enabling innovative applications such as transient filter materials, short-time plant protection materials as well as temporary lightweight materials for aerospace engineering. Thermally and UVtriggerable additives (protected acids or base) have been used in different concentrations to tune the transience performance of the fiber mats over a wide range (75 – 212 °C). The addition of organo-modified clay (OMMT) enhanced mechanical stability and prevented shrinkage at room temperature. Different annealing methods have been used to improve the mechanical properties even further (tensile strength: 2 – 12 MPa, Young’s modulus: 55 – 747 MPa) making these fiber mats attractive for a broad field of applications. An Ashby plot of Young’s modulus versus degradation temperature for electrospun fiber mats is shown, revealing much lower degradation temperatures with higher moduli for PPC composites compared to other electrospun polymers.

1. INTRODUCTION Polymeric materials with a temporary lifetime controlled by a triggerable decomposition process have recently gained significant interest, particularly with regards to clean and straightforward disposal.1–5 Besides this fundamental environmental aspect, transient polymeric materials are being used for resist materials in lithography,6 channel manufacturing for microfluidic devices,7 drug release materials8 and packaging of transient electronics.9 Bacterial or fungal (biodegradable),10,11 solution,12,13 thermal9,14 or UV15,16 treatment can be used to trigger decomposition. Biodegradability is an excellent way for short time consumables like plastic bags, since no special treatment is required as well as the follow-up costs for disposal are nearly zero. However, for many applications biodegradability is unfavorable due its limited


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decomposition speed (at least several hours or even days are required).17 Especially in the case of transient electronics, except for some biomedical devices, a fast and total decomposition is preferred. Research into transient polymers is mainly focused on films or more generally on dense packed polymers. Besides the development of new polymers, increasing the material surface area can also lead to a faster decomposition speed. Therefore it is quite interesting to explore transient porous materials such as foams or nonwovens. The latter can be produced via electrospinning.18 This technique provides access to continuous, long nanofibers with highly oriented polymer chains. The spinning process alters crystallinity and the mechanical properties of the processed polymer.19 There has been extensive work on biodegradable polymeric fiber mats,20–25 but the biodegradation itself is still too slow for many applications. In this work we report novel electrospun transient fiber mats whose decomposition is triggered by UV irradiation and heat treatment. Beside a fast decomposition, these fiber mats also showed excellent mechanical strength. The latter is important for the major application of electrospun nonwovens as filter materials.26 Furthermore, the transient properties demonstrated enables innovative vanishing filters for highly toxic materials. Once the filter is blocked with toxic substances the decomposition is triggered and the filtride including the contaminated washing solution can be collected separately. However, due to the high mechanical strength of these transient fiber mats further applications such as temporary plant protection materials in agriculture, packaging materials and lightweight materials for aerospace engineering can be envisaged. One potential polymer for transient fiber mats is poly(phthalaldehyde) (PPA), which decomposes quickly in the presence of strong acids or heat.27 Although Hernandez et al. showed its potential usage as packaging material for transient electronics,28 the applications of this material in the solid state are limited due to the brittleness of current compositions29. We recently


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demonstrated thin film decomposition of poly(propylene carbonate) (PPC) catalyzed with a thermal acid generator (TAG) or a photo acid generator (PAG) under mild conditions.30 These findings along with the accessibility and mechanical toughness of PPC motivated us to produce transient fiber mats with this material. Inspired by the research of Jung et al. on base-catalyzed hydrolyses of PPC31 we also investigated the effect of a photo base generator (PBG) on the decomposition of PPC in addition to PAG and TAG. The ratio of acid or base generator to PPC has been varied to minimize the degradation temperature of the composite fiber mat. Our studies on transient polycarbonate films imply that the mechanical strength of pure PPC fibers is probably still too low for many applications (e.g. transient filters), although the processing via electrospinning will influence mechanical stiffness due to polymer chain alignment. Fillers such as carbon nanotubes,32 calcium carbonate,33 graphene oxide,34,35 cellulose nanocrystals,20 chitin nanocrystals36 or organo-modified montmorillonite (OMMT)37–39 have been used to enhance stiffness of electrospun polymeric fibers. OMMT worked well for our PPC-composite films and the preparation of the polymer/OMMT composite does not require high temperatures (no release of protected acids/bases). Furthermore, studies on clay reinforced nanofibers with other polymers showed a significant increase in stiffness by exfoliation of the clay among the polymer matrix.40 Consequently OMMT was selected for the first time to improve stiffness and to increase the glass transition temperature (Tg) of PPC in fiber mats. The amount of OMMT was kept to a minimum which guarantees stable fiber mats at room temperature without significantly affecting the decomposition temperature. Composite preparation, electrospinning conditions, as well as post-processing have been modified to tailor the mechanical properties of the PPC-composite fiber mats for the desired applications.


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2. EXPERIMENTAL SECTION 2.1 Materials. Poly(propylene carbonate) (MW ∼ 130.000 Da, Novomer), organo-modified montmorillonite (20 Å, Southern Clay Products), 2,4,5-trichlorobenzenesulfonic acid (TAG, Sigma Aldrich), P-cumenyl(p-tolyl)iodonium tetrakis(pentafluorophenyl)borate (PAG, TCI America), 2-(9-oxoxanthen-2-yl)propionic acid 1,5,7-triazabicyclo[4.4.0]dec-5-ene salt (PBG, TCI America) were used as received. 2.2 Solution Preparation. Solutions of PPC, OMMT and PBG/PAG/TAG were prepared under yellow light using aluminum foil covered glass vials. For the exact compositions please see Table 1 (the percentage values of OMMT and PBG/PAG/TAG in Table 1 are wt% relative to PPC). First PPC and PBG/PAG/TAG were dissolved in 80/20 vol% mixture of dichloromethane and dimethylformamide under shaking at r.t. within 4 h. OMMT was dispersed separately in the same solvent mixture by shaking followed by sonicating with an ultrasonic cell disruptor probe operated at 3 W for 2 min. Finally, both solutions were combined and shaken for additional 12 h to ensure a homogeneous spinning solution. In some cases, the spinning solution was preannealed in a water bath under stirring at 40 °C (please see Table 1 for more details).


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Table 1. Samples with electrospinning conditions. Sample

Spinning Solution* (dissolved in DCM/DMF 80:20)

Voltage

Spinning Temp.

PreAnnealing

PostAnnealing

1 2 3 4 5 6 7 8 9 10 11 12 13

10 wt% PPC, 5% OMMT 10 wt% PPC, 5% OMMT, 5% PAG 10 wt% PPC, 5% OMMT, 10% PAG 10 wt% PPC, 5% OMMT, 15% PAG 10 wt% PPC, 5% OMMT, 5% TAG 10 wt% PPC, 5% PBG 10 wt% PPC, 5% OMMT, 2.5% PBG 10 wt% PPC, 5% OMMT, 5% PBG 10 wt% PPC, 5% OMMT, 10% PBG 10 wt% PPC, 5% OMMT, 15% PBG 10 wt% PPC, 10% OMMT, 5% PBG 10 wt% PPC, 5% OMMT, 5% PBG 10 wt% PPC, 5% OMMT, 5% PBG

14.3 kV 16.5 kV 17.2 kV 17.7 kV 16.7 kV 14.2 kV 13.6 kV 14.1 kV 14.5 kV 14.0 kV 14.5 kV 13.4 kV 14.7 kV 15.9 16.6 kV 15.0 15.7 kV 15.3 16.8 kV 14.8 15.8 kV

20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 20 °C 40 °C with setup 1 42 °C with setup 2 42 °C with setup 2 42 °C with setup 3

no no no no no no no no no no no 2 h at 40 °C 15 h at 40 °C

no no no no no no no no no no no no no

14

10 wt% PPC, 5% OMMT, 5% PBG

No

no

15


No

no

16


15 h at 40 °C

no

17


No

no

18


14.1 kV

20 °C

no

19


14.1 kV

20 °C

no

20


14.1 kV

20 °C

no

21


14.1 kV

20 °C

no

22


14.1 kV

20 °C

no

23


14.1 kV

20 °C

no

24


14.1 kV

20 °C

no

25


14.5 kV

20 °C

no

30 min at 40 °C** 10 min at 50 °C*** 20 min at 50 °C*** 30 min at 50 °C*** 40 min at 50 °C*** 60 min at 50 °C*** 120 min at 50 °C*** 120 min at 50 °C***

* Percentage values of OMMT and PAG/TAG/PBG are wt% relative to PPC. ** The Fiber mat was placed between two thin aluminum plates (to prevent folding during annealing) in a preheated oven at 40 °C. *** Fiber mat remained on the collector plate covered by a thin aluminum plate during post-annealing at 50 °C.


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2.3 Electrospinning Setup. Electrospinning was performed under yellow light or in darkness (after parameters have been adjusted) with a horizontal Spraybase electrospinning setup using a stainless steel flat plate collector. Spinning solution was pumped through a needle (inner diameter = 0.6 mm) with a constant flow rate of 1 ml/h. Each composition was electrospun for 4.5 h to ensure similar fiber mat weight and thickness. The distance between the cannula tip and the collector plate (covered with aluminum foil) was 15 cm. Voltage was adjusted for all compositions to guarantee a good spinning performance (for details please see Table 1). To enable spinning at higher temperatures three different setups were used (see also Table 1). 1. Two IR bulbs (2 x 100 watt) inside the spinning box (distance to collector approx. 17 cm). 2. Setup 1 + flat panel ceramic heater (200 watt, dimensions: 48x2x33 cm) inside spinning box. 3. Two ceramic heat bulbs (2 x 100 watt, distance to collector approx. 17 cm) and flat panel ceramic heater (200 watt, dimensions: 48x2x33 cm) inside spinning box. All setups used a digital outlet heat temperature controller, which automatically switched off and on all devices to guarantee a stable temperature (∆T = 2 °C) inside the spinning box. Furthermore on days with low humidity a humidifier was used and placed close to the spinning setup to keep the humidity level between 30-45% and guarantee good spinning performance. 2.4 Post-Processing. Some of the fiber mats were post-annealed in a pre-heated oven at ≥ 40 °C for different times (for details please see Table 1). 2.5 Setup for Degradation Tests. In a typical degradation test a 1x1 cm fiber mat sample was placed on a glass microscope slide and annealed with a precise adjustable hot plate. In most cases the fiber mat specimen was additionally irradiated with UV light (254 or 365 nm, dose: 2.2 mW/cm2). The distance between the UV light source and the sample was 1 cm. The entire setup was covered in aluminum foil to prevent irradiation by other light sources and to keep the


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air circulation (cooling effect) to a minimum. Both the temperature of the hot plate and the irradiation time have been varied to determine the lowest values for a complete degradation. 2.6 Analytical Methods. IR spectra were recorded with a Thermo Scientific Nicolet iZ10 FTIR with ATR unit with a diamond crystal and OMNIC spectra software version 8.3.103. 1HNMR spectra with 16 scans were captured with a Varian INOVA 400 spectrometer at 400 MHz. TGA traces were recorded on a TA Instruments TGA Q500 using a 10 °C/min rate for heating up to 550 °C. Whereas a heating ramp of 20 °C/min was used for isothermal TGA to reach the desired temperatures. DSC analyses were performed under nitrogen atmosphere with a TA instruments DSC Q2000 and a rate of 10 °C/min for heating and cooling. Both TGA and DSC traces were evaluated with the TA Instruments Universal Analysis 2000 version 4.5A software. SEM micrographs of 3 nm gold sputtered samples were captured on a ZEISS SUPRA 55VP SEM using the ZEISS SmartSEM software with an accelerating voltage of 1.5 kV and the SE2 detector. ImageJ version 1.50 was used to determine the fiber diameters. Tensile testing of ASTM D638 Type V specimens were performed with a TA Instruments Q800 Dynamic Mechanical Analyzer at 23 °C using a constant tensile speed of 1 N/min. Fiber mat thickness was measured with a Mitutoyo IDF-112E Digimatic Indicator. 3. RESULTS AND DISCUSSION All listed compositions in Table 1 have been spun successfully under the mentioned conditions as demonstrated for selected samples in Figure 1. Compositions without PAG or PBG showed very low bead formation and electrospinning itself was sometimes slightly unstable. Whereas compositions with PAG or PBG showed better spinning performance without beads due to a conductivity increase of the spinning solution. Fiber mat thickness was on average ∼ 0.3 ± 0.1 mm. The fiber diameter strongly depended on the spinning solution composition and on the


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spinning parameters (see Table 2 - 5). All fiber mats showed no degradation/weight loss when stored at 18 °C in darkness. In addition no shape change was observed for most fiber mats. Only the fiber mat without clay (sample 6) shrunk under these storage conditions (please see section about enhancing mechanical properties for more details). Please notice that all percentage values of OMMT, PAG, TAG and PBG in this work are weight percent (wt%) relative to PPC.

Figure 1. SEM micrographs of select fiber mats. a) fiber mat with PPC/OMMT (sample 1). b) fiber mat with PPC/OMMT/TAG (sample 5). c) fiber mat with PPC/OMMT/PAG (sample 2). d) fiber mat with PPC/OMMT/PBG (sample 8). 3.1 Optimizing Transience Performance. For applications like transient packaging materials as well as filters for toxic materials it is essential to grantee stability at room temperature but transience after treatment with an external stimulus like heat or UV light. Therefore, thermal and UV-triggerable additives have been tested to initiate polymer chain scission (Scheme 1):

Scheme 1. Transience of a nanofiber after UV and/or heat treatment.


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Our previous work on transient polycarbonate films showed good degradation performance with triggerable acid generators.30 Both a thermal acid generator (TAG) and a photo acid generator (PAG) were also tested here. In addition, the ability for chain scission by a photo base generator (PBG) which releases the strong base triazabicyclodecene (TBD) was investigated. Plausible acid and base catalyzed degradation mechanisms of PPC are shown in Figure 2. A)

B)

Figure 2. A) Acid catalyzed degradation mechanism of PPC (representative for blends with PAG and TAG).41 B) Base catalyzed degradation mechanism of PPC (the used PBG is also a thermal base generator).42


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First, the impact of the fiber composition on the degradation temperature was investigated (no irradiation with UV light). As shown in Table 2 and Figure 3 the addition of TAG, PAG or PBG lowered the degradation temperature of PPC, i.e. the deprotection of PAG and PBG can be also initiated by heating. From all spinning solutions containing 10 wt% PPC and 5% OMMT the one with 5% PBG (sample 8) resulted in the fiber mat with the lowest Tonset (154.9 °C; onset temperature of decomposition is defined by 5% weight loss). In addition this sample showed full degradation at the lowest temperature. That means a strong base catalyzed the decomposition of PPC better than a strong acid. Interestingly higher amounts of PBG than 5% increased the decomposition temperature again. Whereas higher PAG amounts lower the decomposition temperature further (in the investigated range up to 15%). Obviously 5% PBG are enough to catalyze the decomposition in all polymer chains. Most probably an excess of TBD increased again the detected Tonest. No correlation between the fiber diameter and Tonset was observed despite the higher surface area of lower diameter fibers. Apparently the variation of the fiber diameter between different compositions is too small to affect the degradation properties significantly. Table 2. Fiber diameter and Tonset of different unexposed PPC fiber mat compositions. Sample

Amount of Additives*

Fiber Diameter**

Tonset of unexposed fiber mat

1

5% OMMT

261 ± 133 nm

228.2 °C

2

5% OMMT, 5% PAG

185 ± 61 nm

219.6 °C

3

5% OMMT, 10% PAG

244 ± 65 nm

211.7 °C

4

5% OMMT, 15% PAG

241 ± 85 nm

208.7 °C

5

5% OMMT, 5% TAG

220 ± 95 nm

166.4 °C

6

5% PBG

244 ± 110 nm

146.2 °C

7

5% OMMT, 2.5% PBG

300 ± 102 nm

161.3 °C

8

5% OMMT, 5% PBG

248 ± 108 nm

154.9 °C

9

5% OMMT, 10% PBG

388 ± 133 nm

161.1 °C

10

5% OMMT, 15% PBG

336 ± 154 nm

165.6 °C

* Percentage values are wt% relative to PPC. **Evaluated from SEM micrographs. Tonset = onset temperature of decomposition (5% weight loss).


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Figure 3. TGA traces of selected, unexposed PPC-composite fiber mats. All samples shown here contain 5% OMMT. Sample with 5% PBG showed lowest degradation temperature. Second, the impact of UV irradiation on the degradation performance of fiber mats containing PAG or PBG was investigated. Fiber mats have been exposed to UV light (254 nm, dose: 2.2 mW/cm2) for 1.5 h at room temperature. As shown in Figure 4 for samples with 15% PAG and 5% PBG the Tonset decreased after UV irradiation. UV light released either the acid or the base starting chain scission of PPC. This can be seen in Figure 5 for PBG blends by the shape loss of the fibers, whereas SEM micrographs of fiber mats with PAG did not show any significant difference after UV treatment. For both PBG and PAG containing fiber mats the Tonest is even lower for UV irradiation at 50 °C (Figure 4). The further decrease in the decomposition temperature is most probably initiated by a higher diffusion rate at 50 °C of the released acid or base, which triggered a faster chain scission of the polymer chains. As a consequence, PAG and PBG containing fiber mats, which were annealed during UV exposure, transformed into a tacky residue (Figure S1).


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Figure 4. Tonset of unexposed and UV-irradiated (λ = 254 nm) fiber mats with 15% PAG and 5% PBG. UV-exposed fiber mats with lower amounts of PAG or higher amounts of PBG showed increased degradation temperatures. PAG: (Tonset,

UV at r.t.:

160 °C for 10%, 166 °C for 5%;

Tonset, UV at 50 °C : 117 °C for 10%, 141 °C for 5%). PBG: (Tonset, UV at r.t.: 104 °C for 10%, 105 °C for 15%; Tonset, UV at 50 °C : 106 °C for 10%, 90 °C for 15%).

Figure 5. SEM micrographs of a PPC/OMMT/PBG fiber mat (sample 8) a) before and b) after UV irradiation for 1.5 h at room temperature. Based on these results sample 8 was investigated further to determine the threshold values for a complete degradation of PPC. The decomposition of PPC (Figure 6) was detected via a nearly complete weight-loss of the treated samples (since OMMT probably does not degrade under the applied conditions) and via the disappearance of the PPC vibrations in the IR spectrum of the left over material (e.g. the strong CO valence vibration at 1737.45 cm-1 in Figure 7). As a result the


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PPC of a 1x1 cm fiber mat sample (approx. 10 mg) fully degraded under the following conditions (please see supporting information section S2 for TGA traces): • 10 min UV (254 or 365 nm) followed by ∼30 min at 75 °C • 25 min UV (254 or 365 nm) while at 75 °C • 3 min at 165 °C or 5.5 min at 155 °C (no UV required)

Figure 6. Fiber mat before treatment and residue after degradation process. The brittle residue (≤ 0.5 mg) in Figure 6 was identified as OMMT and tiny amounts of PBG (see IR spectra in Figure 7).

Figure 7. IR spectra of residue after treatment, pure OMMT and pure PBG. Most residue signals resulted from OMMT except vibrations at 1660, 1606 and 1314 cm-1, which were assigned to PBG.


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3.2 Enhancing Mechanical Properties. Since the addition of 5% PBG (wt% relative to PPC) resulted in the lowest degradation temperature all studies to enhance the mechanical properties were focused on the PPC/OMMT/PBG blends containing 5% PBG. First, the impact of the composition, namely the amount of OMMT in PPC/OMMT/PBG composites, on the mechanical properties was investigated. The fiber mat blend without OMMT showed the lowest degradation temperature in Table 3 3 (sample 6). However, the mat shrank by ∼20% (from 211 cm2 to 167 cm2) during 3 days storage at 18 °C in darkness (Figure 8), which is unfavorable for most applications. The low Tg (30 °C, see Figure S3-1) of this composition most likely caused the shrinkage.

Figure 8. PPC/PBG fiber mat without clay at room temperature. a) Fiber mat directly after electrospinning. b), c) Shrunken and wrinkled fiber mat after 3 days storage at 18 °C in darkness. The addition of 5% OMMT to the spinning composition increased the Tg of the fiber mat to 36.6 °C (see Figure S3-1) and additionally prevented shrinkage at room temperature. This fiber mat showed a tensile strength of 2.2 ± 0.9 MPa and a Young’s modulus of 54.5 ± 12.0 MPa. In


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addition, the presence of 5% OMMT did not change the onset degradation temperature significantly, only an increase from 146.2 to 154.9 °C was observed (Table 3). The 1H-NMR spectra did not show any solvent residues inside the fiber mats (see representative Figure S4) and thus storage in high vacuum (8.2*10-2 mbar) at r.t. for 12 h did not change the shape, the mechanical properties, or the degradation performance of the fiber mats. Higher loadings of clay (10% OMMT, sample 11 in Table 3) additionally increased the tensile strength (3.0 ± 0.6 MPa) and the Young’s modulus (105.9 ± 26.9 MPa). However the disadvantages of this modification are a higher amount of nondegradable residue and a further increase of the degradation temperature (approx. 15 °C higher than fiber mat with 5% OMMT). Therefore only the composition with 5% OMMT was investigated further. However, for completeness we have to mention that the addition of PBG, which is required to lower the degradation temperature of PPC, slightly decreased the mechanical properties (please see sample 1 and 8 in Table 3). A plausible reason for this could be that PBG can interact with the organo-modified surface of the clay reducing the interaction with the polymer chains. This might also explain why we observed with high OMMT loading (10%) a further improvement of the mechanical properties in comparison to known PVA-OMMT40 or PS-OMMT43 fiber mats.


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Table 3. Tensile Strength and Young’s modulus of PPC/OMMT/PBG fiber mats in dependence of their composition.

Sample

Amount of Additives*

Tonset of unexposed fiber mats / °C

Fiber Diameter / nm

Tensile Strength / MPa

Young’s Modulus / MPa

6

5% PBG

146.2

244 ± 110

4.1 ± 0.2

123.9 ± 4.3

1

5% OMMT

228.8

261 ± 133

2.8 ± 0.1

77.7 ± 1.5

8

5% OMMT, 5% PBG

154.9

248 ± 108

2.2 ± 0.9

54.5 ± 12.0

11

10% OMMT, 5% PBG

169.4

146 ± 60

3.0 ± 0.6

105 ± 26.9

Tonset = onset temperature of decomposition (5% weight loss), measured with TGA. Fiber diameter evaluated from SEM micrographs. Tensile strength and Young’s modulus evaluated from stress strain curves (see section S6 in supporting information). Sample 6 showed better mechanical properties, due to adhesion of fibers at their overlap points (see Figure S5-1) caused by the low Tg of this composition. * Percentage values are wt% relative to PPC. During tensile testing the exposure time to non-yellow light was kept as short as possible. Second, the impact of different annealing treatments on the mechanical properties of PPC/OMMT/PBG fiber mats containing 5% OMMT and 5% PBG was investigated (Table 4). Based on our previous work on transient polycarbonate composite films30 we expected a further increase of the Young’s modulus by annealing, due to an improvement of the clay/polymer interactions. Therefore we tested pre-annealing, spinning at higher temperatures and postannealing. DSC traces revealed a Tg at 35 °C for the used PPC granulate and a phase transition at 36 °C for the pure OMMT (see Figure S3-2 and S3-3). Consequently a minimum temperature of 40 °C was chosen for all annealing processes to achieve an impact on the mechanical properties.


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Table 4. Tensile Strength and Young’s of PPC/OMMT/PBG fiber mats (same composition) in dependence of different annealing treatments.

Sample Spinning Temp.

PreAnnealing

Post-Annealing

Fiber Tensile Young’s Diameter / Strength / Modulus / nm MPa MPa

8

20 °C

no

no

248 ± 108

12

20 °C

2 h at 40 °C

no

13

20 °C

15 h at 40 °C

no

14

40 °C with setup 1

No

15

42 °C with setup 2

16

2.2 ± 0.9

54.5 ± 12.0

240 ± 105

2.8 ± 1.0

85.7 ± 7.3

207 ± 73

3.8 ± 0.1

86.7 ± 3.3

no

143 ± 63

4.7 ± 0.1

121.7 ± 6.4

No

no

340 ± 110

4.6 ± 0.5

120.6 ± 3.0

42 °C with setup 2

15 h at 40 °C

no

274 ± 111

6.1 ± 0.4

144.9 ± 0.5

17

42 °C with setup 3

No

no

263 ± 87

8.2 ± 0.1

255.4 ± 17.5

18

20 °C

no

30 min at 40 °C

404 ± 146

8.2 ± 0.1

316.9 ± 2.0

Samples have been electrospun from a 10 wt% PPC, 5% OMMT*, 5% PBG* (*wt% relative to PPC) composition. During tensile testing the exposure time to non-yellow light was kept as short as possible. Tensile strength and Young’s modulus evaluated from stress strain curves (see section S6 in supporting information). Fiber diameter evaluated from SEM micrographs. Pre-annealing of the spinning solution increased both tensile strength to 2.8 ± 1.0 MPa (27% increase) and the Young’s modulus to approx. 86 MPa (65% increase) see sample 12 in Table 4. Extending the pre-annealing time to 15 h did not increase the E-modulus further. SEM micrographs showed a higher tendency of fiber branching compared to non-annealed samples (Figure 9).

Figure 9. SEM micrographs of selected fiber mats. a) Micrograph of pre-annealed sample 12 (2 h at 40 °C). b) Micrograph of non-pre-annealed sample 17 which was spun at 42 °C (setup 3). Electrospinning at higher temperatures also enhanced the mechanical properties (see Table 4 and Figure 10). Three different arrangements were tested as described in the experimental


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section. Fiber mats spun at 40 °C (IR bulbs, setup 1) and 42 °C (IR bulbs and flat panel heater, setup 2) showed Young’s moduli of approx. 121 MPa which is higher compared to fiber mats spun with pre-annealed spinning solutions. The combination of a pre-annealed spinning solution with spinning at 42 °C (setup 2) just slightly increased the Young’s modulus to 145 MPa. Whereas electrospinning at 42 °C (no pre-annealing) with ceramic heat lamps in combination with a flat panel heater (setup 3) enhanced the Young’s modulus of the fiber mat by a factor of 2 (255 MPa) compared setups with IR bulbs (see sample 15 and 17 in Table 4). The reason for the doubling is probably that the ceramic heat lamps also heated the air inside the spinning box and not primarily the irradiated surface (collector plate). This enables a pre-annealing of the spinning solution inside the needle and a faster evaporation of the solvent during the spinning. Under these conditions, the thermal time constant of the collector plate was increased, providing evidence for our speculation. Moreover, as can be seen in Table 4 and 5 there is no clear correlation between the fiber diameter and the Young’s modulus. This is not surprising since the fiber diameters vary substantially in all fiber mats. Whereas a strong correlation between the tensile strength and the Young’s was observed as expected, i.e. the tensile strength increased by the same factors as the Young’s modulus for all above mentioned samples. However, at these high temperatures electrospinning itself became really unstable (probably due to the low boiling point of the solvent DCM and due to a decrease of the humidity with time). As such, we did not try to optimize the conditions further even if fiber mats with low bead formation (see exemplary Figure 9b) and improved mechanical properties were produced.


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Figure 10. Illustration of enhanced mechanical properties of a PPC/OMMT/PBG fiber mat by electrospinning at higher temperatures. For stress-strain curves of selected specimens please see Figure S6-1. Post-annealing was performed in a pre-heated oven. Preliminary experiments showed strong shrinkage and folding of the fiber mat when fiber mats were annealed ≥ 35 °C (Figure 11). Postannealing while fiber mats were placed between two thin aluminum plates prevented folding, but shrinkage was not completely eliminated. However, using this technique at 40 °C for 30 min resulted in a strong increase of both tensile strength (8.2 ± 0.1 MPa, 3.7 fold increase) and Young’s modulus (316.9 ± 2.0 MPa, 5.8 fold increase) compared to untreated fiber mats. In comparison to the other annealing techniques this method resulted in the best mechanical properties so far (see Figure 12). Furthermore, no film formation was observed, even if the applied temperature with 40 °C was above the Tg.


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Figure 11. SEM micrographs and images of sample 8 a) non-annealed and b) post-annealed for 10 min at 50 °C.

Figure 12. Tensile strength and Young's modulus comparison of different annealing methods. For stress-strain curves of selected specimens please see Figure S6-2. The strong increase of the mechanical properties due to post-annealing could result either from shrinkage or from the phase transition of the clay in combination with the low Tg of PPC. For clarification, shrinkage has to be eliminated during post-annealing. Shrinkage as well as folding were prevented when fiber mats were post-annealed while still adhering to the aluminum foil wrapped collector plate. Placing in addition a thin aluminum plate on the top of the sample resulted in even better mechanical properties. Obviously, the small adhesion between the mat and the aluminum foil is strong enough to avoid shrinkage and the slight pressure by the aluminum plate during annealing enhanced stiffness further (see Table 5). It is most likely that


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the phase transition of the clay in combination with the Tg of PPC caused the fibers to stick together at their overlapping points and thus enhanced the mechanical properties of the fiber mats. The latter is confirmed by SEM micrographs (Figure 13). Although the Tg of this fiber mat composition (36.3 °C, see Figure S3-1) was below the annealing temperature no film formation could be observed if the annealing time was kept ≤ 30 min (Figure 13). Table 5. Tensile Strength and Young’s modulus of PPC/OMMT/PBG fiber mats in dependence of post-annealing time.

Sample

Spinning Temp.

PreAnnealing

Post-Annealing

Fiber Diameter / nm

Tensile Young’s Strength / Modulus / MPa MPa

8

20 °C

no

no

248 ± 108

2.2 ± 0.9

54.5 ± 12.0

19

20 °C

no

10 min at 50 °C

326 ± 160

4.7 ± 0.1

189.2 ± 74.0

20

20 °C

no

20 min at 50 °C

233 ± 104

6.4 ± 1.7

303.0 ± 27.4

21

20 °C

no

30 min at 50 °C

316 ± 160

22 23 24 25*

20 °C 20 °C 20 °C 20 °C

no no no no

40 min at 50 °C 60 min at 50 °C 120 min at 50 °C 120 min at 50 °C

315 ± 179 film formation film formation

11.6 ± 1.5 10.7 ± 2.4 7.4 ± 1.9 7.8 ± 0.3

746.8 ± 114.1 546.4 ± 72.8 418.7 ± 49.4 449.1 ± 4.0

380 ± 172

8.8 ± 0.1

546.1 ± 27.7

Most of the samples have been spun from a 10 wt% PPC, 5% OMMT, 5% PBG composition except sample 25* (10% instead of 5% OMMT). Fiber mats remained on collector plate during annealing to prevent shrinkage. Tensile strength and Young’s modulus evaluated from stress strain curves (see section S6 in supporting information). Fiber diameter evaluated from SEM micrographs.


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Figure 13. SEM micrographs of 50 °C post-annealed PPC/OMMT/PBG fiber mats. Postannealing time increases from a) 10 min, b) 20 min, c) 30 min, d) 40 min, e) 60 min to f) 120 min. As shown in Figure 14, the tensile strength and Young’s modulus strongly increased with annealing time until a maximum value is reached after 30 min at 50 °C (tensile strength ∼ 12 MPa and Young’s modulus ∼ 747 MPa). Longer annealing times at 50 °C resulted in a decrease of the mechanical properties until they became stable ≥ 60 min (7-8 MPa for tensile strength and ∼ 418-449 MPa). As mentioned earlier, SEM micrographs showed film formation for annealing times longer than 30 minutes (Figure 13). This means film formation decreased the mechanical properties again. The loss of mechanical strength due to film formation most probably resulted from a decrease in the degree of order of the polymer chains (induced by electrospinning) and a loss of exfoliation of the clay in combination with its agglomeration. In any case, a tensile strength of 12 MPa and a Young’s modulus of 747 MPa after 30 min post-annealing are quite remarkable, since these values are much higher than the values which


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we observed previously for PPC/OMMT films (tensile strength: ∼ 6 MPa, Young’s modulus: 534 MPa)30. Taking into consideration that the addition of PBG even negatively effects the mechanical properties as mentioned before (see sample 1 and 8 in Table 3) and that much less material is required for a low density fiber mat as compared to a densely packed film, these results become even more remarkable. In addition, the post-annealed fiber mat with annealing time ≤ 30 min showed the same degradation performance as a similar untreated one (please see Figure S2-1 and S2-2). Whereas longer annealing times enhanced the thermostability of the nanocomposite due to film formation; e.g. 1 h post-annealing resulted in a 9 °C increase of Tonset (164 °C).

Figure 14. Tensile strength and Young's modulus of post-annealed fiber mats. The composition of all fiber mats was identical. For stress-strain curves of selected specimens please see Figure S6-3. In a preliminary experiment we investigated briefly if a doubling of the amount of clay could prevent film formation and may result in a higher stiffness like the non-annealed fiber mats. As a result after 120 min post-annealing both the tensile strength (8.8 ± 0.1 MPa) and Young’s


24

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modulus (546.1 ± 27.7 MPa) are slightly higher than the values for a fiber mat with only 5% OMMT after same annealing time. However, even if this composition has a higher Tg (38.2 °C, see Figure S3-1) film formation could only be reduced but not prevented (Figure S5-2). Certainly less film formation caused a decreasing tendency of clay agglomeration and also a lower loss of order of the polymer chains compared to 120 min post-annealed fiber mats with 5% OMMT. Overall these observations are in good alignment with the earlier described results, but as also mentioned earlier higher amounts of clay increased the onset degradation temperatures as well as the amount of nondegradable residue. As a consequence, mechanical properties are tunable by annealing time for a specific fiber mat composition as well as by the added amount of OMMT. Depending on the transient application both can be used for a precise adjustment of the mechanical properties. However, the adjustability by post-annealing alone is a great benefit for the large scale production, since the electrospinning parameters, like voltage and flow rate have to be optimized for just one specific sample composition. 3.3 Superior properties of PPC/OMMT/PBG fiber mats. The PPC/OMMT/PBG composite fiber mats presented here show a substantially higher stiffness at low(er) decomposition temperatures compared to other electrospun fiber mats as demonstrated in an Ashby plot (Figure 15). This plot shows the unique combination of high mechanical performance and mild decomposition conditions not yet achieved in other materials. Of course, fiber mats of high performance polymers like polyimide (6F-PI)44 exhibit higher Young’s moduli but only in combination with very high decomposition temperatures. Otherwise, in vitro degradation of biodegradable polymeric fiber mats in PBS solution (e.g. PCL/Maxon)45 starts already at 37 °C. However, beside the fact that PPC is also biodegradable46 (not investigated here) the degradation rate (i.e. only 11% weight loss within 42 days were detected for PCL/Maxon fiber mats)45 as


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Page 26 of 52

well as the Young’s modulus are considered too low for applications like transient packaging materials or transient filters. Furthermore the tuneability of stiffness and decomposition temperature by external stimuli as well as by changing the ratio of components enables access to a huge unexplored area of materials as shown in the Ashby plot. Taking into consideration that the thermal properties of the fiber mats can also be shifted by replacing PBG with TAG or PAG, as demonstrated earlier in this paper, the application field will expand further.

Figure 13. Ashby plot of Young's modulus as a function of degradation temperature for electrospun fiber mats. PPC/OMMT/PBG fiber mats cover a huge area of materials with high stiffness and low degradation temperature. Other electrospun fiber mats have a substantial lower stiffness and/or a higher degradation temperature. Please see Table S7 in the supporting information for the exact data of the reference materials. 4. CONCLUSION In conclusion, blends from poly(propylene carbonate) with organo-modified montmorillonite (OMMT) and a thermal and UV-triggerable additive, which release either a strong acid or base, have been electrospun successfully. The use of 5 wt% (relative to PPC) photo base generator,


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which releases the strong base TBD, reduces the thermostability of poly(propylene carbonate) significantly and enables the transience of PPC under relatively mild conditions. Fiber mats which have been irradiated with UV light (254 or 365 nm, for either 10 min before annealing or the entire time during annealing) showed full decomposition within 25-30 minutes at 75 °C. Besides this behavior, the addition of OMMT enhances the mechanical stability and prevents shrinkage at room temperature. Both tensile strength and Young’s modulus were improved by pre-annealing of the spinning composition, electrospinning at ≥ 40 °C as well as post-annealing of the fiber mats. Post-annealing showed so far the best improvement. A 30 min post-annealing at 50 °C while fiber mat remained on the collector plate covered by a thin aluminum plate showed no shrinkage or folding and led to a tensile strength of ∼ 12 MPa and Young’s modulus of ∼747 MPa. Experiments without the aluminum plate revealed that the slight pressure by the plate improved stiffness further due to stronger adhesion of the fibers at their overlapping points. However, annealing in any form (except post-annealing with longer annealing times than 30 min) did not significantly change the degradation performance and enabled tunable mechanical properties of the transient PPC-composite fiber mats. Consequently these transient composite fiber mats are suitable for a wide range of applications in the field of transient packaging materials, transient plant protection material, transient filter systems for toxic materials, and any application where a lightweight transient material with good mechanical strength is needed (e.g. in aerospace engineering).

ASSOCIATED CONTENT Supporting Information. Illustration of the transformation of a PPC/OMMT/PBG fiber mat into a tacky residue after UV


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irradiation at 50 °C. TGA and DSC traces, 1H-NMR spectrum, additional SEM micrographs and stress-strain curves of selected samples. Data of reference materials for the Ashby plot. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT This research was primarily supported by the Defense Advanced Research Project Agency (DARPA). This work was performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-1542081). This work made use of the Cornell Center for Materials Research Shared Facilities which are supported through the NSF MRSEC program (DMR-1120296). The authors would like to thank M. Burgard from the University of Bayreuth for some valuable advice regarding electrospinning and Novomer (especially S. Allen, now at Aramco Services Company) for providing PPC. REFERENCES (1) Patrick, J. F.; Robb, M. J.; Sottos, N. R.; Moore, J. S.; White, S. R. Polymers with Autonomous Life-Cycle Control. Nature 2016, 540, 363–370. (2) Phillips, O.; Schwartz, J. M.; Kohl, P. A. Thermal Decomposition of Poly(propylene carbonate): End-capping, Additives, and Solvent Effects. Polym. Degrad. Stab. 2016, 125, 129– 139.


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(43) Ji, Y.; Li, B.; Ge, S.; Sokolov, J. C.; Rafailovich, M. H. Structure and Nanomechanical Characterization of Electrospun PS/Clay Nanocomposite Fibers. Langmuir 2006, 22, 1321–1328. (44) Cheng, C.; Chen, J.; Chen, F.; Hu, P.; Wu, X.-F.; Reneker, D. H.; Hou, H. High-Strength and High-Toughness Polyimide Nanofibers: Synthesis and Characterization. J. Appl. Polym. Science 2010, 116, 1581–1586. (45) Schindler, C.; Williams, B. L.; Patel, H. N.; Thomas, V.; Dean, D. R. Electrospun Polycaprolactone/Polyglyconate Blends: Miscibility, Mechanical Behavior, and Degradation. Polymer 2013, 54, 6824–6833. (46) Ma, X.; Chang, P. R.; Yu, J.; Wang, N. Preparation and Properties of Biodegradable Poly(propylene carbonate)/Thermoplastic Dried Starch Composites. Carbohydr. Polym. 2008, 71, 229–234.


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For Table of Contents Graphic use only


35


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

254x170mm (150 x 150 DPI)


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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


182x102mm (300 x 300 DPI)



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

185x63mm (300 x 300 DPI)


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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


247x176mm (300 x 300 DPI)



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

214x179mm (300 x 300 DPI)


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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


254x84mm (150 x 150 DPI)



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

197x102mm (150 x 150 DPI)


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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


216x174mm (300 x 300 DPI)



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

168x171mm (150 x 150 DPI)


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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


254x84mm (150 x 150 DPI)



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

210x198mm (300 x 300 DPI)


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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


254x84mm (150 x 150 DPI)



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

210x184mm (300 x 300 DPI)


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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


211x183mm (300 x 300 DPI)



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

254x256mm (150 x 150 DPI)


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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


201x167mm (150 x 150 DPI)



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

153x79mm (150 x 150 DPI)


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Transient Fiber Mats of Electrospun Poly ... - ACS Publications

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