Tailoring the interfaces in glass fiber-reinforced photopolymer composites

Accepted Manuscript Tailoring the interfaces in glass fiber-reinforced photopolymer composites Melahat Sahin, Sandra Schlögl, Gerhard Kalinka, Jieping Wang, Baris Kaynak, Inge Mühlbacher, Wolfgang Ziegler, Wolfgang Kern, Hansjörg Grützmacher PII:

S0032-3861(18)30225-8

DOI:

10.1016/j.polymer.2018.03.020

Reference:

JPOL 20433

To appear in:

Polymer

Received Date: 13 January 2018 Revised Date:

8 March 2018

Accepted Date: 9 March 2018

Please cite this article as: Sahin M, Schlögl S, Kalinka G, Wang J, Kaynak B, Mühlbacher I, Ziegler W, Kern W, Grützmacher Hansjö, Tailoring the interfaces in glass fiber-reinforced photopolymer composites, Polymer (2018), doi: 10.1016/j.polymer.2018.03.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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GRAPHICAL ABSTRACT Tailoring the interfaces in glass fiber-reinforced photopolymer composites

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Melahat Sahin, Sandra Schlögl*, Gerhard Kalinka, Jieping Wang, Baris Kaynak, Inge Mühlbacher, Wolfgang Ziegler, Wolfgang Kern, Hansjörg Grützmacher

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Tailoring the interfaces in glass fiber-reinforced photopolymer composites Melahat Sahina, Sandra Schlögla,*, Gerhard Kalinkab, Jieping Wangc, Baris Kaynakd, Inge

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Mühlbachera, Wolfgang Zieglerd, Wolfgang Kerna,d, Hansjörg Grützmacherc a

Polymer Competence Center Leoben GmbH, A-8700 Leoben, Austria

b

BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin,

Germany c

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Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland

d

Chair in Chemistry of Polymeric Materials, Montanuniversitaet Leoben, A-8700 Leoben, Austria

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Correspondence to: Sandra Schlögl (E-mail: [email protected])

ABSTRACT

The present work provides a comparative study on the interface and adhesion properties of surface modified single glass fibers embedded in an acrylate matrix. To facilitate a covalent bonding at the fiber-

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matrix interface, the fibers are functionalized with selected organosilanes that comprise either passive (unsaturated C=C bonds of methacrylate moieties) or photoactive functionalities (photocleavable bis(acyl)phosphane oxide groups). Immobilization of the functional silanes is carried out by a classic silanization reaction involving a condensation reaction across the surface hydroxyl groups of the

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inorganic glass fibers. The change of the physico-chemical properties of the fibers due to desizing and subsequent surface modification is monitored by X-ray photoelectron spectroscopy and zeta potential measurements. In addition, scanning electron microscopy is used to follow the changes in surface

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morphology. After the modification step, the desized and modified single fibers are embedded in a photocurable acrylate resin formulation. By performing single fiber pull-out tests, maximum pull-out force, friction strength and apparent interfacial shear strength are determined as a function of the coupled silanes. The results reveal that the attached organosilanes lead to a significant increase in adhesion strength, whilst the performance of the photo-cleavable organosilane is superior to the passive methacryl-functional derivative.

KEYWORDS: photopolymer composites; fiber-matrix interface; single fiber pull-out test; surface modification; photocleavable organosilanes

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INTRODUCTION Benefiting from high stiffness at low weight, polymer based composites have conquered the market in numerous applications ranging from automobile and aircraft to marine and sports

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industry.[1–3] Along with the employment of classic thermosetting resins, current research is geared towards the fabrication of photopolymer composites using photocurable resins as polymer matrices.[4] Whilst light triggered reactions have several advantages such as curing at low temperature and temporal and spatial control of the reaction, they suffer from a low

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penetration depth of the incident light.[5,6] Thus, only thin layers can be efficiently crosslinked or polymerized. In particular, for the fabrication of thick photopolymer composites multilayer

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processes are required.[7] Recently, Allonas and co-workers demonstrated that the layer-bylayer build-up of photopolymer composites leads to homogenous monomer conversion through the depth of the layered materials.[8] In addition, they revealed that oxygen inhibition does not significantly influence the mechanical properties of the composites. Going beyond conventional manufacturing routes, photopolymerization reactions have gained increased attention in additive manufacturing techniques.[9–11] Particularly, digital light processing has paved the

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way towards an easy scaling and fast processing of photoreactive materials, since one layer of photopolymer is fabricated during one-time of projection.[12] Another promising approach is the continuous liquid interface production where complex 3D objects are produced at rates of

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hundreds of millimeters per hour.[13]

Prominent applications of photopolymer composites are dental fillings and anterior crown restorations, where an acrylic matrix is reinforced with inorganic fillers to obtain the desired

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mechanical stiffness, wear resistance and polishability retention.[14] Along with silica[15] or zirconia[16] particles, inorganic fibers (e.g. short E-glass fibers)[17,18], ceramic nanofibers (e.g. zirconia–silica)[19] and nanotubes (e.g. titania)[20] are exploited as reinforcing fillers. In terms of processing, appropriate wetting of the filler by the resin and sufficient adhesion between filler and polymer-based matrix are required to ensure a good mechanical performance of the composite materials. Numerous studies describe the important role of the filler-matrix interface on the static and dynamic mechanical properties as well as the environmental resistance of polymer based composites.[21–24] To adjust the interface

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properties, various modification methods are described to change the chemical and physical properties of organic and inorganic fillers. With respect to glass fibers, established surface modification routes involve plasma techniques[25] and wet chemical approaches such as

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coating or the covalent attachment of selected functional coupling agents.[26] In particular, silanization reactions have become a prominent approach to tailor glass fiber surfaces.[27] In the absence of a catalyst or a protic solvent, the reaction mechanism involves a hydrolysis of the organosilane by water that is pre-adsorbed on the glass fiber surface. The hydrolyzed silane forms hydrogen bonds with the surface silanol groups on the fiber surface leading to a

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reversible immobilization of the organosilanes. A covalent bonding of the silanes is obtained by a subsequent thermal annealing step at elevated temperature, which induces the formation of

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stable siloxane bonds via a condensation reaction. In photopolymer composites, fibers and fillers are often treated with (3-methacryloxypropyl) trimethoxysilane as it enables a covalent binding to the (meth)acrylic polymer matrix.[28] Sideridou demonstrated that both storage modulus as well as loss modulus increased by the application of methacryl-functionalized silica particles in methacrylate resin formulations.[29]

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Going beyond the attachment of organosilanes with passive functions (i.e. the chemical structure does not change by exposure to an external stimulus), the current work focuses on the modification of glass fibers with organosilanes that undergo photo-triggered Norrish Type I reactions. In recent work we developed a new synthesis route towards the preparation of a

a

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bis(acyl)phosphane (BAPO) derivative with a terminal trimethoxysilane group.[30] By following phospha-Michael-addition

reaction

TMESI2-BAPO

was

obtained

from

a

stable

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bis(mesitoyl)phosphane and 3-(trimethoxysilyl)propyl methacrylate. Due to the trimethoxysilyl group, TMESI2-BAPO can be easily immobilized onto glass, metal oxide and silicate surfaces by the condensation reaction with the surface hydroxyl groups.[30] BAPO derivatives are typically used as photoinitiators in photo-induced polymerization and crosslinking reactions as they undergo a Norrish Type I reaction upon light exposure.[31,32] The light induced cleavage of the σ bond between the carbonyl and the phosphane oxide group yields a phosphinoyl-benzoyl radical pair, which initiates photopolymerization reactions.[33–35] BAPO absorbs in the long wavelength region (λ = up to 440 nm), whilst the cleavage products

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are UV transparent. This makes BAPO an ideal initiator for the photo-curing of dental fillings.[34,36] Recently, we grafted TMESI2-BAPO onto the surface of nano-sized silica and exploited the

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photoactive particles for the initiation of the radical-mediated thiol-ene reaction.[37] Rapid curing and high conversion yields of the monomers were observed confirming the photoactivity of the attached organosilane. In a further work, we also demonstrated that TMESI2-BAPO modified silica nanoparticles display a low extractability, which indicates that they are covalently anchored within the thiol-ene matrix.[38]

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In the present work, we transfer the concept to fiber surfaces and study the influence of the photoactive organosilanes on the properties of the fiber-matrix interface in photopolymer

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composites. Single-fiber pull-out tests are performed to evaluate the adhesion characteristics involving maximum pull-out force, apparent interfacial shear strength and friction strength. The results are then compared to desized glass fibers, and to fibers modified with the conventional (3-methacryloxypropyl) trimethoxysilane.

Materials and Chemicals

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EXPERIMENTAL

Sized glass fibers (StarRov®907, 9600 tex) with a filament diameter of 32 µm were provided by

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Johns Manville (Colorado, US). The photoactive silane 3-(trimethoxysilyl)propyl 3-[bis(2,4,6trimethylbenzoyl) phosphinoyl]-2-methyl-propionate (TMESI2-BAPO) was synthetized as

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previously reported.[30] Irgacure 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide, BAPO) was provided by BASF (Ludwigshafen, Germany). Ammonium hydroxide solution (30 %) was purchased from Roth, and hydrogen peroxide (30 %) was obtained from VWR chemicals. (3-methacryloxypropyl) trimethoxysilane (GF 31) was supplied by Wacker Chemie (Burghausen, Germany). Di(trimethylolpropane) tetraacrylate and all other chemicals were supplied by Sigma-Aldrich (St. Louis, US) and were used without further purifications. Desizing of Glass Fibers

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The organic sizing of the glass fibers was removed by a three-step cleaning procedure. Firstly, part of the sizing was removed by a Soxhlet extraction with acetone. For the extraction, around 10 g of the glass fibers were placed into a cellulose extraction thimble (Mn: 645, inner diameter:

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33 mm and height: 94 mm) and extracted with 250 mL of acetone (77 °C for 24 h). The fibers were then removed and dried at 120 °C. In the second step, the residual sizing was removed by placing the extracted fibers in a piranha solution, which consisted of one equivalent of 30 % aqueous H2O2 and one equivalent 95 % H2SO4. After the treatment, the glass fibers were withdrawn from the acidic piranha solution and repeatedly rinsed with deionized water. In the

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last cleaning step, the glass fibers were treated with a basic piranha solution, which consisted of one equivalent of 30 % aqueous H2O2 and one equivalent 30 % aqueous NH4OH at 50-60 °C for

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20 min. The glass fibers were then repeatedly rinsed with deionized water, and subsequently dried for 12 h at 120 °C.

Surface Modification of Desized Glass Fibers

The desized glass fibers were placed into a 1 % (w/v) solution of either TMESI2-BAPO or GF 31 in anhydrous toluene at 60 °C. After 24 h, the fibers were taken from the solution and dried at

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120 °C for 2 h. The modified fibers were then rinsed for several times with acetone to remove any physically attached organosilanes.

Surface Characterization of Glass Fibers

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XPS analysis of glass fibers prior to and after each desizing step as well as after surface modification was performed with a K-Alpha photoelectron spectrometer (Thermo Scientific, US)

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equipped with an Al-Kα X-ray source (hν = 1486.6 eV) and a hemispherical analyzer. The survey scan was carried out with a pass energy of 200 eV and an energy step size of 1.0 eV, whilst high resolution spectra were recorded with a pass energy of 20 eV and an energy step size 0.1 eV. For the measurement of TMESI2-BAPO modified fibers the spot size was 400 µm, and for all the other fiber surfaces a spot size of 300 µm was used. The peaks were fitted using a Gaussian/Lorentzian mixed function employing Shirley background correction (Software Thermo Avantage v5.906, Thermo Scientific). All analyses were performed at room temperature. Hydrogen was omitted in the calculation of the elemental composition.

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Zeta potential measurements were carried out with an Electrokinetic Analyser (EKA, Anton Paar KG, Graz, Austria) to determine the isoelectric point. The streaming potential method was used to determine the zeta potential of the desized and modified glass fibers in 1 mM KCl. The zeta

using an autotitrating unit (RTU, Anton Paar KG, Graz, Austria).

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potential was measured starting from natural pH to lower acidic values by adding 50 mM HCl

Scanning electron microscopy (SEM) images were taken with a scanning microscope Tescan Vega II (Tescan Orsay Holding, Brno) applying a working voltage of 15 kV.

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Preparation and Characterization of Photopolymer Matrix

1 wt% Irgacure 819 was dissolved into di(trimethylolpropane) tetraacrylate by stirring the resin

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formulation at room temperature for 12 h. The kinetics of the free radical photopolymerization was monitored by FT-IR spectroscopy using a Vertex 70 spectrometer (Bruker, United States). 16 scans were accumulated with a resolution of 4 cm-1 and the area of the absorption peaks was calculated with OPUS software. For sample preparation, the resin formulation was dropcast between two CaF2 discs and illuminated with a light emitting diode (LED) lamp (zgood® wireless LED curing lamp) under N2 atmosphere. The light intensity of the LED amounted to

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4.89 mW/cm2 (λ = 420-450 nm) and the kinetics of the acrylate photopolymerization was monitored during 6 min exposure.

Characterization of Fiber-Matrix Interactions

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Maximum pull-out force (Fmax) and apparent interfacial shear strength (τapp) were obtained from single fiber pull-out tests as a measure of the adhesion between the photopolymer matrix

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and selected glass fiber surfaces. The pull-out tests were carried out with a homemade apparatus. For sample preparation, a single glass fiber was cut to a length of 10 mm with a scalpel and fixed to a steel fiber holder with cyanoacrylate glue. A defined volume of the photoreactive resin formulation was placed in a small hole (d = 1 mm) in an aluminum sample carrier with a syringe. The perpendicularly aligned fiber was then embedded with varying length (approx. 30-250 µm) in the resin droplet by using a homemade apparatus (see Fig. S1 in supporting information).[40,41] The resin was photochemically cured under argon atmosphere

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using the LED lamp (zgood® wireless LED curing lamp) as light source. The distance between sample surface and LED lamp amounted to 1 cm and the exposure time was 4 min. After photocuring, the embedded fiber was cut from the steel fiber holder and the free part of

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the fiber was glued onto a screw platform, which was attached to a force transducer. The free length of the fiber between the surface of the matrix and the screw platform ranged between 10 and 30 µm. First, the aluminum block was attached to the actuator and then, the free part of the fiber was attached to the force transducer. With a ramp speed of 1.0 µm s-1, time, force and displacement of the fixed fiber were recorded. The resolution of the used force transducer was

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better than 1 mN.

After the pull-out, the diameter of the embedded fiber, the pull-out zone and the pulled-out

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fiber surface were characterized by a 3D laser scanning microscope (VK-X100, Keyence, Japan). It should be noted that only the tests where the sample exhibited a pull-out due to the failure of the interface between fiber and matrix were used for further data processing. For each fiber surface, ten measurements were performed at different embedding lengths.

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RESULTS AND DISCUSSION

To adjust the adhesion between glass fibers and an acrylic matrix, functional silanes bearing either a methacrylate or a bis(acyl)phosphane oxide group were immobilized on the surface of the fibers. These silanes were chosen since they enable a covalent bonding of the modified

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fiber surface to the acrylic resin during the photo-induced curing step. In terms of the attached methacrylate function, bonding is accomplished during the free radical induced crosslinking of

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the acrylate monomers (“grafting-through” mechanism) (see Fig. 1a). In contrast, fibers modified with the synthesized TMESI2-BAPO undergo Norrish Type I cleavage reactions forming a phosphinoyl-benzoyl radical pair, which is able to take part in the initiation of the free radical crosslinking

reaction

of

the

multi-functional

acrylate

monomers

(“grafting-from”

mechanism).[33] In a second reaction step, the formation of the polymer matrix may proceed by the following mechanism: R2P•(=O) + H2C=CH-CO2R -> R2PO-CH2-CH•-CO2R (see Fig. 1b). In this process the glass fiber is covalently bonded to and embedded in the growing polymer matrix. Moreover, the immobilized phosphinoyl radicals may also abstract hydrogen atoms

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from components of the resin mixture to give fiber-P(=O)HR groups (R = polymer chain or COMes). Subsequently, a phospha-Michael-addition according to fiber-P(=O)HR + R2C=CH-CO2R -> fiber-P(=O)R-CR2-CH2-CO2R) may additionally lead to a covalent bonding between the

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modified fiber and the acrylate matrix.[39]

Figure 1 – Simplified reaction schemes for coupling of (a) GF 31 and (b) TMESI2-BAPO modified glass fiber surfaces to an acrylic matrix during free radical polymerization of a multi-functional

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acrylate. Note that all acrylates with group R will be eventually polymerized leading to a highly crosslinked polymer matrix. The radical centers in the growing chains will be terminated by H abstraction reactions and/or radical recombinations. Preparation and Characterization of Desized Glass Fibers For the surface modification experiments, commercially available glass fibers with surface sizing were used. Sizings are typically applied by the producer as an aid in processing and to protect the reinforcing fibers both from damage during processing as well as from corrosion during

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storage. Sizings contain coupling agents (e.g. silanes) and other additives such as film forming agents (e.g. starch, polyvinyl alcohol), lubricants and antistatic components.[40,41] Coupling agents are attached to the fiber surface during application of the sizing from solution and they

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play a significant role in enhancing the mechanical properties of fiber-reinforced composite by covalent and/or other physical-chemical interactions with the polymer matrix.[42,43] In contrast, other additives, which are usually not covalently bound to the fiber surface, are mainly responsible for protecting the fiber surface from mechanical damage and to facilitate infiltration of the fiber bundle by the polymer melt or by the resins during processing.[41]

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Since glass fibers with proprietary sizings were employed in the current work, a three-step cleaning procedure was carried out to remove the organic components. The change in the

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chemical surface composition of the fiber after each cleaning step was characterized by XPS spectroscopy. The surface composition of the sized glass fiber prior to and after the cleaning procedure involving an acetone extraction and a treatment with acidic and basic piranha solution is given in Table 1. On the surface of the sized glass fiber, silicon oxide (Si–O), at 103.5 eV, and organic silicon (C–Si–O), at 101.8 eV, are observed, which indicates the presence

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of a silane based coupling agent. Moreover, carbon (76.2 At.%) is detected, which is associated with C-linked groups in the organic additives of the sizing.[44–46] Fig. 2a shows the deconvoluted high-resolution XPS Si 2p spectra of the sized glass fiber. Neither calcium (Ca) nor aluminum (Al) was detected on the surface, which suggests that the thickness of the sizing is

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above 10 nm (probing depth of XPS), as Ca and Al are both a component of the inorganic glass fiber.[47] This is in good agreement with previous work, which states that the thickness of

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applied sizings is typically in the range of 100 nm.[41] After acetone extraction the carbon content (72.1 At.%) slightly decreased whilst the oxygen content (20.4 At.%) increased, suggesting that the amount of non-covalently bound organic additives was reduced by this cleaning step. By employing the acidic piranha treatment, the carbon content was significantly decreased by further 20 At.%. Additionally, the silicon and oxygen content on the surface increased, which indicates a removal of the sizing to a certain extent. An elemental analysis from the XPS survey spectra shows the presence of small amounts of sulfur on the glass fiber surface after acidic piranha treatment. As the acidic piranha

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solution contains H2SO4, the results give an indication that a small amount of the sulfuric acid has been physisorbed onto the glass surface during the chemical etching.[48] During the subsequent basic piranha treatment of the glass fiber, the carbon content

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(36.8 At.%) further decreased corresponding to an increasing oxygen content (37.7 At.%.) Furthermore, only silicon oxide at 103.5 eV was observed in the Si 2p spectra with an O/Si ratio close to two. This is an expected value for silicon dioxide (glass) without sizing agent (see Fig. 2b).[44,45] These results confirm that the sizing agent has been completely removed from the glass fiber surface during the applied cleaning steps although the piranha treatment seems to

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be the more efficient one to reduce the components of the sizings.[47] The detected carbon content is attributed to physisorbed atmospheric hydrocarbon, as the basic piranha treatment

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increases the surface energy of glass fibers.[49] In addition, no sulfur was detected after the basic treatment step, which suggests that the adsorbed sulfuric groups have been removed from the surface during the alkaline etching step.

To verify the efficiency of the applied cleaning procedure, an alternative method of desizing of glass fibers was applied, which involves burning off the sizing. Comparing the two procedures, it

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is obvious that both methods result in a similar carbon, oxygen and silicon atom content (see Table 1).

Table 1 - Chemical composition of sized glass fiber surfaces prior to and after the three-step

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cleaning procedure involving acetone extraction (1st step), acidic piranha treatment (2nd step) and basic piranha treatment (3rd step) as derived from XPS data. In a reference experiment, the

Fiber

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sizing was burned off at 565 °C for 2 h under air. Relative Atom Concentration (At.%) C

O

Si

Ca

Al

S

Sized glass fiber

76.2

16.5

7.3

–

–

–

Acetone extracted

72.1

20.4

7.5

–

–

–

Acidic piranha treated

52.4

30.4

12.1

3.0

1.0

1.1

Basic piranha treated

36.8

37.7

17.0

4.2

4.3

–

Burned off

37.2

38.3

19.6

3.3

1.6

-

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(b) Intensity (C/s)

Intensity (C/s)

Si-C

Si-O

106

104

102

100

98

108

106

104

102

100

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108

Si-O

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Binding energy (eV)

Binding energy (eV)

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Figure 2 – High-resolution Si 2p XPS spectra of sized glass fiber (a) prior to and (b) after the three-step cleaning procedure involving acetone extraction and acidic and basic piranha treatment.

Along with XPS measurements, zeta potential experiments were performed to study the

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changes in surface polarity in dependence on the applied cleaning step (see Fig. 3a). Prior to the cleaning step, the isoelectric point (IEP) of the sized fibers amounts to 5.31 indicating the presence of basic groups from the applied sizing.[50] After cleaning with acidic piranha solution, the IEP shifts to lower values (3.32), which suggests that part of the organic sizing is removed

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and the acidic silanol moieties of the inorganic glass fiber become the dominating groups of the outermost surface. After the subsequent basic piranha treatment, this effect is intensified,

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since the residual silanol groups are divested of the covering sizing and the IEP drops to 2.45. The negative zeta potential values are typical for glass surfaces, as the acidic surface groups are fully dissociated in the basic pH range and form negative surface charges.[51] Whilst the surface charges of the inorganic fibers mainly rely on the protonation and deprotonation of the silanol groups, also other elements on the fiber surface (e.g. Al-OH, Ca-OH) contribute to them.[52] The results are in good agreement with the XPS measurements and confirm the desizing of the fiber during the applied cleaning step. However, it is interesting to note, that burning off the organic sizing at 565 °C does lead to an IEP (2.92), which is significantly higher than the IEP of

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the basic piranha treated fiber surface. On the one hand, this might be related to traces of the sizing, which have not been removed during the burning step. One the other hand, it should be regarded that the a thermal treatment of the glass fibers leads to a dehydroxylation of geminal

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and vicinal silanol groups, which occurs between 200 and 600 °C.[53] Thus, the amount of acidic silanol groups on the surface is reduced leading to a shift of the IEP to a higher pH value. This correlates well with previous work on the surface properties of E-glass fibers, which demonstrated that the density of the surface hydroxyl groups decreases from 2.29 ± 0.044 to

40

40

(a)

2

3

4

5

6

-20 -40 -60

7

zeta potential (mV)

0

20 0

2

3

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zeta potential (mV)

20

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1.71 ± 0.03 OH nm-1 by a thermal treatment at 600 °C.[52]

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(b)

5

6

7

-20 -40 -60

-80

-80

pH value (-)

pH value (-)

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Figure 3 - Zeta potential as a function of the pH value of (a) untreated sized glass fibers (solid squares), cleaned by acetone extraction and acidic (open triangles) and subsequent alkaline

piranha treatment (open squares), and cleaned by burning at 565 °C for 2 h under air (open circles). Zeta potential as a function of the pH value of (b) glass fibers cleaned by the applied

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three-step procedure (open squares) in comparison to TMESI2-BAPO (open triangles) and GF 31

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modified glass fibers (open diamonds).

Since the desizing step may damage the fiber, the morphology of the fibers after each cleaning step was studied by scanning electron microscopy (SEM). Sized fibers exhibit a rough and inhomogeneous surface that is mainly attributed to the applied organic coating (see Fig. 4a). The surface becomes smoother over the extraction with acetone (see Fig. 4b) and the treatment with the acidic piranha solution (see Fig. 4c), which confirms the partial removal of components of the sizing. A smooth fiber surface without any traces of the sizing is observed

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after the treatment with the alkaline piranha solution (see Fig. 4d). It should be noted that the cleaned fibers do not exhibit any surface damages. With respect to burned off fibers, the surface morphology exhibits randomly scattered residues

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of the sizing (see Fig. 4e). From the results it can be concluded, that the chemical etching process is more efficient in the removal of the sizing than the thermal treatment under the

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applied reaction conditions.

Figure 4 – SEM micrographs of (a) untreated sized glass fiber s, fibers after (b) acetone extraction and subsequent cleaning with (c) acidic and (d) subsequent alkaline piranha treatment, (e) fibers after burning off the sizing, and desized fibers after modification with (f) GF 31 and (g) TMESI2-BAPO.

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Preparation and Characterization of Modified Glass Fibers The surface functionalization was carried out with glass fibers desized via the three-step cleaning procedure, since SEM micrographs revealed that they are free of any traces of the

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sizing. In addition, zeta-potential measurements indicate that the etched glass fiber surface contains a higher number of free silanol groups, which is beneficial for the immobilization of the functional organosilanes. In particular, a trialkoxysilyl-functionalized derivative of BAPO, which was synthesized as previously reported,[30] and a commercially available trialkoxysilyl-

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functional methacrylate were employed as reactive coupling agents. It should be noted, that oligolayers can be formed during the modification step as the organosilanes contain three hydrolyzable groups.

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The covalent attachment of the reactive organosilanes was confirmed by XPS spectroscopy and the chemical composition of the basic piranha treated glass fiber prior to and after the surface modification with TMESI2-BAPO and GF 31 is given in Table 2. Deconvoluted high-resolution C 1s and Si 2p spectra of the fibers with the coupled organosilanes are shown in Fig. 5a and b, respectively. The C 1s spectra of fibers modified with either TMESI2-BAPO or GF 31 exhibit one

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main peak at 284.7 eV, which is assigned to hydrocarbon (C–C) groups of the alkyl chain and the aromatic moieties.[54–56] Other fitted peaks are associated to the C–O signal at 286.2 eV, and the C=O signal at 288.7 eV.[55–57]

The Si 2p high-resolution XPS spectra of the functionalized fiber surfaces comprise two

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characteristic functionalities: Si–O at 103.5 eV, and C–Si–O at 101.7 eV[44–46], which are attributed to the inorganic glass fiber surface and to the attached organic trimethoxysilyl

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groups (C–Si–O), respectively.[58] Additionally, a deconvoluted P 2p spectrum of the TMESI2BAPO modified fiber was taken (see Fig. 6) and two P peaks are observed: P–C at 132.3 eV, and P–O at 133.5 eV, which are in good agreement with the chemical structure of the molecule TMESI2-BAPO.[59]

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(b)

C-O C=O

290

288

C-C

Intensity (C/s)

Intensity (C/s)

C-C

286

284

282

280

C-O C=O

292

290

Binding energy (eV)

(d)

105

104

Intensity (C/s)

103

102

284

282

280

Si-C

101

100

99

Binding energy (eV)

Si-O

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Intensity (C/s)

Si-C

Si-O

106

286

Binding energy (eV)

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(c)

288

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(a)

98

106

105

104

103

102

101

100

99

98

Binding energy (eV)

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Figure 5 - Deconvolution of high-resolution (a,b) C 1s and (c,d) Si 2p XPS spectra of (a,c) TMESI2BAPO and (b,d) GF 31 modified glass fiber surfaces.

P-C

AC C

EP

Intensity (C/s)

P-O

142

140

138

136

134

132

130

128

126

Binding energy (eV)

Figure 6 - Deconvoluted high-resolution P 2p XPS spectra of a TMESI2-BAPO modified glass fiber surface.

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Table 2 Chemical surface composition of basic piranha treated glass fibers prior to and after the modification with TMESI2-BAPO and GF31, as derived from XPS data. Relative Concentration (At.%) Fiber O

Si

Desized[1]

36.8

37.7

17.0

GF 31

55.7

28.1

9.9

TMESI2-BAPO

72.1

19.8

4.1

Al

P

4.2

4.3

–

4.2

2.0

–

2.0

–

2.0

Desizing was accomplished with a 3-step procedure; acetone extraction (1st step), acidic piranha treatment (2nd step) and basic piranha treatment (3rd step)

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[1]

Ca

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C

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The modified fibers were further investigated by zeta potential measurements (see Fig. 3b). The coupling of the organosilanes leads to a slight shift of the IEP to higher pH values: from 2.45 to 2.58 for GF 31 modified fibers, and to 2.65 for TMESI2-BAPO modified fibers. Since both organosilanes do not contain any basic or acidic groups, the change of the IEP is mainly related to a change in the density of the free acidic silanol groups at the glass fibers surface: silanol groups are partly consumed by the condensation reaction with the organosilanes.

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Along with the chemical surface composition, the modification step also changes the surface morphology of the fibers. From SEM micrographs it can be seen, that fibers functionalized with GF 31 have an inhomogeneous surface structure (see Fig. 4f). The inhomogeneity can be related

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to grafted oligolayers formed by the condensation reaction of the trialkoxysilyl-functionalized organosilane. These inhomogeneous structures are even more pronounced in fiber surfaces

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modified with TMESI2-BAPO (see Fig. 4g). Although the modification with GF 31 and TMESI2BAPO was carried out under the same reaction conditions, the SEM micrographs indicate that fibers modified with TMESI2-BAPO exhibit a higher modification degree. Characterization of the Fiber-Matrix Adhesion The influence of the attached functional groups on the adhesion strength of the interface between the glass fibers (desized versus modified) and a photosensitive resin formulation was characterized by single fiber pull-out tests. Prior to the experiments, the reaction kinetics and maximum conversion of the visible light induced curing of the acrylic resin were determined by

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FT-IR spectroscopy. The conversion of the acrylic moieties as a function of the exposure dose is shown in Fig. S2 (see supporting information). The final monomer conversion is rather low and does not exceed 50 %, which is typical for the free radical induced crosslinking of multi-

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functional acrylates. The curing reaction follows a chain-growth mechanism and the gel point occurs at low conversions, which hinders the diffusion of the monomers and limits the final monomer conversion.[60] Based on the FT-IR experiments, an exposure time of 4 min was used for the photo-curing of the acrylic resin to ensure maximum monomer conversion of the photopolymer matrix in the pull-out measurements.

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Figure 7 displays the force-displacement curves for desized and modified glass fibers embedded with a comparable embedding length in the photo-cured acrylate resin. A typical pull-out curve

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comprises four stages.[61–63] In the first stage, a linear load-displacement relationship is observed until the initiation of debonding. In this stage, the free part of the glass fiber is stretched purely elastically, the interface is still intact and the adhesion energy resists to the applied pull-out forces. The second stage involves a crack initiation and a stable crack propagation. Energy is dissipated by both the creation of new surfaces as well as friction

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between the crack planes. Furthermore, crack grows along the interface since the shear stress at the surface of the embedded fiber is higher than the bond strength. The crack growth is restricted by the amount of energy, which is able to supply the crack grow. The source of the required surface energy and friction energy is the elastic deformation of the sample, mainly in

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the free part of the fiber. If there is not enough energy for driving the crack forward, the crack propagation stops until more external energy is given by moving the pulling actuator. The third

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stage is characterized by an unstable crack growth with a sudden drop in the load. In this stage, the existing elastic energy in the sample is more than enough to drive the crack forward to its end.

After complete debonding of the interface, only frictional sliding is observed in the fourth stage. In this state, with further pull-out, the interfacial friction force decreases in relation to the remaining fiber surface area, which is still in contact with the polymer matrix. A stick-slip effect sometimes occurs indicating that there is still some level of adhesion between the completely debonded fiber and the polymer.

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With respect to desized fibers, the slope of the force-displacement plots did not change significantly until maximum load. The debonding seems to lead immediately to an abrupt decrease in the pull-out force, which does not decrease to 0 but to 70 % of its maximum value

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(see Fig. 7a). In contrast to desized fibers, the force-displacement curves of GF 31 modified fibers show a different behaviour after the first stage (see Fig. 7b). After the first drop of the force, areas within the interface seem to be newly adhered, and a strong stick-slip effect is observed. In some cases the combination of frictional stress and adhesion in the already chemical debonded

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interface could be higher exceeding the level of initial debonding.[64]

The modification of fibers with TMESI2-BAPO has a different effect on the load response of

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single fiber pull-out tests (see Fig. 7c). Contrary to desized fibers and fibers modified with GF 31, the force-displacement plots of fibers modified with TMESI2-BAPO clearly follow the previously described four stages of a fiber pull-out experiment. However, during the frictional sliding state

0,6

(a)

OH OH OH OH

(b)

(c)

O Si H H O H O

O

H O H O Si H O

O

O O O

O

P

Mes Mes

O

EP

0,2

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Force (N)

0,4

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a pronounced stick-slip effect is observed.

0,0

0

10

20

Displacement (µm)

30

0

10

20

Displacement (µm)

30

0

10

20

30

Displacement (µm)

Figure 7 – Influence of the surface composition of (a) desized, (b) GF 31 and (c) TMESI2-BAPO modified glass fibers on the force-displacement curves for single glass fibers embedded in a photo-cured acrylate matrix with a comparable embedding length (170–180 µm).

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Force-displacement plots of desized and modified fibers embedded in the photo-cured acrylate matrix with varying embedding length are provided in the supporting information (see Fig. S3 and S4). At varying embedding lengths, the pull-out curves of desized and modified fibers follow the same stages as described above.

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In a further step, the apparent interfacial shear strength (τapp) was estimated by τapp = Fmax/A where Fmax corresponds to the maximum pull-out force and A to the embedded fiber surface. A can be easily calculated as follows: A = dfiber*π*le where dfiber is the fiber diameter and le is the

embedding length which were both measured by laser scanning microscopy.[65,66] The

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estimation of τapp using this method is a rough estimation, since the formula presume an uniform stress along the embedded fiber (“apparent” interfacial shear strength). A crack

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growth process would lead to an inhomogeneous stress field. Note that quantitative determination of the adhesive strength was only carried out if complete fiber-matrix debonding (not a break of the fiber) occurred, which was evaluated by optical microscopy. In case of an adhesive failure, the micrographs exhibited relatively smooth and clean glass fibers with a small resin droplet at the end of the embedding length comprising a small meniscus, where the fiber

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and the surface to the droplet met. In terms of cohesive failure, a part of the matrix would remain on the glass fiber even after the pull-out of the fiber against friction. In Fig. 8a-c the maximum pull-out force (before the first drop) is plotted as a function of the embedded matrix area, in which the gradient can be related to τapp. The slope of the surface

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modified fibers is significantly steeper than the one of the desized fiber, which can be attributed to a higher τapp and thus, a higher adhesive strength at the fiber-matrix interface.

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This effect is even more pronounced for fibers modified with TMESI2-BAPO. Since SEM measurements showed a higher modification degree of fibers modified with TMESI2-BAPO, the enhanced adhesive strength might be explained by a higher number of covalent bonding between the attached organosilane and the polymer matrix. In addition, it should be noted that phosphorus radicals are more reactive than acyl radicals[33] and that the phosphorus atom of the attached TMESI2-BAPO can serve as cross-linking point[67] (see Fig. 1b), which may further contribute to an improved adhesion strength of the fibre-matrix interface.

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However, the adhesive strength can be also influenced by a different curing degree of the acrylate at the interface. Previous work revealed that immobilized TMESI2-BAPO is able to initiate radical induced polymerization reactions by efficiently undergoing a Norrish Type I

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cleavage.[38] In photo-induced radical reactions, the initiation rate (Ri) is directly proportional to the concentration of the initiating species [PI], the quantum yield of the photoinitiation (Φ), the intensity of the incident light (I0) and the molar extinction coefficient of the initiator (ε): Ri = 2 Φ I0 ε [PI]. [68],[69] At a higher radical concentration at the interface both a higher polymerization rate and an enhanced final conversion of the monomers is obtained at the fiber-

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matrix interface.

(a)

(b)

0,8 0,6 0,4

y=10.45x

y=16.06x

0,2 0,0 0,01

(c)

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Max. Pull-out Force, Fmax (N)

1,0

0,01

0,02

2

Embedded Fiber Area (mm )

0,02

0,03

0,04 2

Embedded Fiber Area (mm )

y=26.46x

0,01

0,02

0,03

0,04

Embedded Fiber Area (mm2)

EP

Figure 8 - Maximum pull-out force (obtained before the first drop of the load) as a function of embedded fiber area of (a) desized, (b) GF 31 and (c) TMESI2-BAPO modified glass fibers.

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By plotting the apparent interfacial shear strength versus the embedded matrix length, information on the fracture behavior of the fiber-matrix interface is obtained (see Fig. 9a-c). From these results, it can be concluded that the interfacial shear strength of the investigated fibers is independent of the embedded fiber length. This behavior suggests that the failure of the fiber/matrix interface is accomplished by a ductile-type deformation, leading to a uniform stress distribution along the embedded part of the fiber. A pronounced influence of a sharp growing crack would lead to an inhomogeneous stress field and thus, to a strong dependence of the “apparent interfacial shear strength” of the embedding length.[70]

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40

(a)

(b)

(c)

35

25

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Stress (MPa)

30

20 15 10 5

100

200

300

400

Embedding Length (µm)

100

200

300

SC

0 400

Embedding Length (µm)

100

200

300

400

Embedding Length (µm)

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Figure 9 - Apparent interfacial shear strength (full circles) and friction strength (open circles) as a function of embedding length of (a) desized, (b) GF 31 and (c) TMESI2-BAPO modified glass fibers.

In addition, a statistical analysis of the data was performed to verify the significance of the differences in the obtained apparent interfacial shear strengths. In particular, in a first step a

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one factorial variance analysis was carried out with a significance level set at p = 0.05. Based on the result it can be assumed, that the experimental pull-out test data of the individual different interfaces have unequal variances. Consequently, in a second step a two-sample unequal variance t-test was chosen for the comparison of the mean value. According to the t-test, the

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apparent interfacial shear strength of desized as well as GF 31 and TMESI2-BAPO modified glass fibers is different confirming the positive effect of the surface modification on the fiber-matrix

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adhesion in single fiber photopolymer composites. Box plots of the apparent interfacial shear strength of the investigated fiber surfaces are presented in the supporting information (see Fig. S5).

Along with the apparent interfacial shear strength, the friction strength was calculated and plotted as a function of the embedding length (see Fig. 9a-c). For desized fibers, the frictional strength is approximately linear proportional to the embedding length, whilst the frictional strength does not follow a clear trend after the surface modification. Comparing the frictional strength of the three different fiber surfaces, it is obvious, that fibers modified with GF 31 (5.9 ±

21

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2.4 MPa) and TMESI2-BAPO (7.3 ± 2.4 MPa), respectively, exhibit a higher friction than the desized fibers (3.2 ± 2.1 MPa). Statistical analysis of the data revealed that the friction strength of desized and surface modified fibers differ significantly, whilst the friction force of fibers

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modified with GF 31 and TMESI2-BAPO is not different according to the t-test. Box plots of the friction strength of desized and modified fiber surfaces are provided in the supporting information (see Fig. S6).

CONCLUSIONS

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The surface of glass fibers was modified by immobilizing photoreactive (TMESI2-BAPO) and methacryl-functional organosilanes to enhance the adhesion strength towards acrylate based

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photopolymers. The modification of desized glass fibers with GF 31 and TMESI2-BAPO was evidenced by zeta potential (shift of the IEP from 2.45 to 2.58 and 2.65) and XPS experiments (increase in carbon content and depletion of the Si-O peak at 103.5 eV). SEM experiments of modified fibers revealed the formation of grafted oligolayers during the modification step, which were more distinctive in TMESI2-BAPO modified surfaces than in GF 31 modified ones.

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The attached functionalities were expected to undergo a covalent bonding to an acrylate matrix during the free radical induced photopolymerization of the monomers. Single fiber pull-out tests showed that surface modification facilitates a distinctive increase in maximum pull-out force and apparent interfacial shear strength corresponding to a stronger fiber-matrix

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adhesion. Glass fibers modified with TMESI2-BAPO proved to be superior, which was attributed to the formation of a higher number of covalent bonds (due to a higher modification degree).

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For the functionalized fibers, the apparent interfacial shear strength was independent of the embedded fiber length, which indicates a ductile-type failure of the fiber-matrix interface. The results clearly showed that the fiber-matrix adhesion was conveniently adjusted and improved by appropriate surface functionalization of the glass fibers.

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ACKNOWLEDGEMENTS

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This study was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET K1 program and the “Produktion der Zukunft” program of the Austrian Federal Ministry for Transport, Innovation and Technology and of the Austrian Federal Ministry for Economy, Family and Youth with contributions of Montanuniversitaet

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Leoben. PCCL is funded by the Austrian Government and the State Governments of Styria and Lower Austria. The authors thank Peter Fuchs (PCCL) for his support on statistical data analysis

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and interpretation of the results. We also thank the ETH Zürich for financial support.

REFERENCES

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EP

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[1] Pendhari SS, Kant T, Desai YM. Application of polymer composites in civil construction: A general review. Composite Structures 2008;84:114–24. [2] Koronis G, Silva A, Fontul M. Green composites: A review of adequate materials for automotive applications. Composites Part B: Engineering 2013;44:120–7. [3] Mouritz AP, Bannister MK, Falzon PJ, Leong KH. Review of applications for advanced threedimensional fibre textile composites. Composites Part A: Applied Science and Manufacturing 1999;30:1445–61. [4] Endruweit A, Johnson MS, Long AC. Curing of composite components by ultraviolet radiation: A review. Polym. Compos. 2006;27:119–28. [5] Stansbury JW. Curing Dental Resins and Composites by Photopolymerization. J Esthet Restor Dent 2000;12:300–8. [6] Leprince JG, Palin WM, Hadis MA, Devaux J, Leloup G. Progress in dimethacrylate-based dental composite technology and curing efficiency. Dental materials official publication of the Academy of Dental Materials 2013;29:139–56. [7] Peck JA, Li G, Pang S, Stubblefield MA. Light intensity effect on UV cured FRP coupled composite pipe joints. Composite Structures 2004;64:539–46. [8] Pierrel J, Ibrahim A, Croutxé-Barghorn C, Allonas X. Effect of the oxygen affected layer in multilayered photopolymers. Polym. Chem. 2017;8:4596–602. [9] Stansbury JW, Idacavage MJ. 3D printing with polymers: Challenges among expanding options and opportunities. Dental materials official publication of the Academy of Dental Materials 2016;32:54–64.

23

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[10] Zarek M, Layani M, Cooperstein I, Sachyani E, Cohn D, Magdassi S. 3D Printing of Shape Memory Polymers for Flexible Electronic Devices. Advanced materials (Deerfield Beach, Fla.) 2016;28:4449–54. [11] Ligon SC, Liska R, Stampfl J, Gurr M, Mülhaupt R. Polymers for 3D Printing and Customized Additive Manufacturing. Chem. Rev. 2017;117:10212–90. [12] Wang X, Jiang M, Zhou Z, Gou J, Hui D. 3D printing of polymer matrix composites: A review and prospective. Composites Part B: Engineering 2017;110:442–58. [13] Tumbleston JR, Shirvanyants D, Ermoshkin N, Janusziewicz R, Johnson AR, Kelly D et al. Additive manufacturing. Continuous liquid interface production of 3D objects. Science (New York, N.Y.) 2015;347:1349–52. [14] Bijelic-Donova J, Garoushi S, Vallittu PK, Lassila, Lippo V J. Mechanical properties, fracture resistance, and fatigue limits of short fiber reinforced dental composite resin. The Journal of prosthetic dentistry 2016;115:95–102. [15] Karabela MM, Sideridou ID. Synthesis and study of properties of dental resin composites with different nanosilica particles size. Dental materials official publication of the Academy of Dental Materials 2011;27:825–35. [16] Hambire UV, Tripathi VK. Optimisation of Compressive Strength in Zirconia Nanoclusters of the Bis-GMA & TEGDMA based Dental Composites. Procedia Engineering 2013;51:494–500. [17] Garoushi S, Vallittu PK, Lassila, Lippo V J. Short glass fiber reinforced restorative composite resin with semi-inter penetrating polymer network matrix. Dental materials official publication of the Academy of Dental Materials 2007;23:1356–62. [18] Khan AS, Azam MT, Khan M, Mian SA, Ur Rehman I. An update on glass fiber dental restorative composites: a systematic review. Materials science & engineering. C, Materials for biological applications 2015;47:26–39. [19] Guo G, Fan Y, Zhang J, Hagan JL, Xu X. Novel dental composites reinforced with zirconiasilica ceramic nanofibers. Dental materials official publication of the Academy of Dental Materials 2012;28:360–8. [20] Khaled, S M Z, Miron RJ, Hamilton DW, Charpentier PA, Rizkalla AS. Reinforcement of resin based cement with titania nanotubes. Dental materials official publication of the Academy of Dental Materials 2010;26:169–78. [21] Amdjadi, P., Ghasemi, A., Najafi, F., Nojehdehian, H. Pivotal role of filler/matrix interface in dental composites. Biomedical Research 2017;3:1054–65. [22] Sideridou ID, Karabela MM. Effect of the structure of silane-coupling agent on dynamic mechanical properties of dental resin-nanocomposites. J. Appl. Polym. Sci. 2008;110:507– 16. [23] Wilson KS, Allen AJ, Washburn NR, Antonucci JM. Interphase effects in dental nanocomposites investigated by small-angle neutron scattering. Journal of biomedical materials research. Part A 2007;81:113–23.

24

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[24] Wilson KS, Antonucci JM. Interphase structure-property relationships in thermoset dimethacrylate nanocomposites. Dental materials official publication of the Academy of Dental Materials 2006;22:995–1001. [25] Lim K, Lee D. Surface modification of glass and glass fibres by plasma surface treatment. Surf. Interface Anal. 2004;36:254–8. [26] Liu Z, Zhang L, Yu E, Ying Z, Zhang Y, Liu X et al. Modification of Glass Fiber Surface and Glass Fiber Reinforced Polymer Composites Challenges and Opportunities: From Organic Chemistry Perspective. COC 2015;19:991–1010. [27] Plueddemann EP. Chemistry of Silane Coupling Agents. In: Plueddemann EP, editor. Silane Coupling Agents. Boston, MA: Springer US; 1991, p. 31–54. [28] Lung, Christie Ying Kei, Matinlinna JP. Aspects of silane coupling agents and surface conditioning in dentistry: an overview. Dental materials official publication of the Academy of Dental Materials 2012;28:467–77. [29] Sideridou ID, Karabela MM. Effect of the amount of 3-methacyloxypropyltrimethoxysilane coupling agent on physical properties of dental resin nanocomposites. Dental materials official publication of the Academy of Dental Materials 2009;25:1315–24. [30] Sangermano M, Periolatto M, Castellino M, Wang J, Dietliker K, Grützmacher JL et al. A Simple Preparation of Photoactive Glass Surfaces Allowing Coatings via the “Grafting-from” Method. ACS applied materials & interfaces 2016;8:19764–71. [31] Radl SV, Schipfer C, Kaiser S, Moser A, Kaynak B, Kern W et al. Photo-responsive thiol–ene networks for the design of switchable polymer patterns. Polym. Chem. 2017;8:1562–72. [32] Sahin M, Ayalur-Karunakaran S, Manhart J, Wolfahrt M, Kern W, Schlögl S. Thiol-Ene versus Binary Thiol-Acrylate Chemistry: Material Properties and Network Characteristics of Photopolymers. Adv. Eng. Mater. 2016. [33] Jockusch S, Turro NJ. Phosphinoyl Radicals: Structure and Reactivity. A Laser Flash Photolysis and Time-Resolved ESR Investigation. J. Am. Chem. Soc. 1998;120:11773–7. [34] Albuquerque, Pedro Paulo A C, Moreira, Allana D L, Moraes RR, Cavalcante LM, Schneider, Luis Felipe J. Color stability, conversion, water sorption and solubility of dental composites formulated with different photoinitiator systems. J Dent 2013;41 Suppl 3:e67-72. [35] Decker C. Photoinitiated crosslinking polymerisation. Prog. Polym. Sci. 1996;21:593–650. [36] Ikemura K, Endo T. A review of the development of radical photopolymerization initiators used for designing light-curing dental adhesives and resin composites. Dental Materials Journal 2010;29:481–501. [37] Sahin M, Schlögl S, Kaiser S, Kern W, Wang J, Grützmacher H. Efficient initiation of radicalmediated thiol-ene chemistry with photoactive silica particles. J. Polym. Sci. A Polym. Chem. 2017:894–902.

25

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[38] Sahin M, Krawczyk KK, Roszkowski P, Wang J, Kaynak B, Kern W et al. Photoactive silica nanoparticles: Influence of surface functionalization on migration and kinetics of radicalinduced photopolymerization reactions. Eur. Polym. J. 2018;98:430–8. [39] Kawaguchi S, Nomoto A, Sonoda M, Ogawa A. Photoinduced hydrophosphinylation of alkenes with diphenylphosphine oxide. Tetrahedron Letters 2009;50:624–6. [40] Jones FR. A Review of Interphase Formation and Design in Fibre-Reinforced Composites. J. Adhes. Sci. Technol. 2010;24:171–202. [41] Blau PJ (ed.). ASM handbook. 10th ed. Materials Park, OH: ASM International; 2001. [42] Gao S, Mäder E. Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites. Composites Part A: Applied Science and Manufacturing 2002;33:559–76. [43] Zhang RL, Huang YD, Su D, Liu L, Tang YR. Influence of sizing molecular weight on the properties of carbon fibers and its composites. Materials & Design 2012;34:649–54. [44] Veres M, Koós M, Tóth S, Füle M, Pócsik I, Tóth A et al. Characterisation of a-C: H and oxygen-containing Si:C:H films by Raman spectroscopy and XPS. Diamond and Related Materials 2005;14:1051–6. [45] O'Hare L, Parbhoo B, Leadley SR. Development of a methodology for XPS curve-fitting of the Si 2p core level of siloxane materials. Surf. Interface Anal. 2004;36:1427–34. [46] Önneby C, Pantano CG. Silicon oxycarbide formation on SiC surfaces and at the SiC/SiO2 interface. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 1997;15:1597–602. [47] Feih S, Wei J, Kingshott P, Sørensen BF. The influence of fibre sizing on the strength and fracture toughness of glass fibre composites. Composites Part A: Applied Science and Manufacturing 2005;36:245–55. [48] Jang HK, Chung YD, Whangbo SW, Lee YS, Lyo IW, Whang CN et al. Effects of chemical etching with sulfuric acid on glass surface. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 2000;18:401–4. [49] Lee G, Lee N, Jang J, Lee K, Nam J. Effects of surface modification on the resin-transfer moulding (RTM) of glass-fibre/unsaturated-polyester composites. Composites Science and Technology 2002;62:9–16. [50] Jacobasch H. Surface phenomena at polymers. Makromolekulare Chemie. Macromolecular Symposia 1993;75:99–113. [51] Bismarck A, Boccaccini AR, Egia-Ajuriagojeaskoa E, Hülsenberg D, Leutbecher T. Surface characterization of glass fibers made from silicate waste: Zeta-potential and contact angle measurements. J Mater Sci 2004;39:401–12. [52] Mittal KL (ed.). Silanes and other coupling agents. Leiden: VSP; 2009.

26

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[53] Kim JM, Chang SM, Kong SM, Kim K, Kim J, Kim W. Control of hydroxyl group content in silica particle synthesized by the sol-precipitation process. Ceramics International 2009;35:1015–9. [54] Roppolo I, Chiappone A, Bejtka K, Celasco E, Chiodoni A, Giorgis F et al. A powerful tool for graphene functionalization: Benzophenone mediated UV-grafting. Carbon 2014;77:226–35. [55] Sow C, Riedl B, Blanchet P. Kinetic studies of UV-waterborne nanocomposite formulations with nanoalumina and nanosilica. Progress in Organic Coatings 2010;67:188–94. [56] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558–65. [57] Özçam AE, Roskov KE, Spontak RJ, Genzer J. Generation of functional PET microfibers through surface-initiated polymerization. J. Mater. Chem. 2012;22:5855. [58] Sangermano M, Periolatto M, Castellino M, Wang J, Dietliker K, Grützmacher JL et al. A Simple Preparation of Photoactive Glass Surfaces Allowing Coatings via the "Grafting-from" Method. ACS applied materials & interfaces 2016;8:19764–71. [59] Niu F, Tao L, Deng Y, Wang Q, Song W. Phosphorus doped graphene nanosheets for room temperature NH3 sensing. New J. Chem. 2014;38:2269. [60] Andrzejewska E. Photopolymerization kinetics of multifunctional monomers. Prog. Polym. Sci. 2001;26:605–65. [61] Sockalingam S, Nilakantan G. Fiber-Matrix Interface Characterization through the Microbond Test. International Journal of Aeronautical and Space Sciences 2012;13:282–95. [62] Zhandarov S, Mäder E. Peak force as function of the embedded length in pull-out and microbond tests: effect of specimen geometry. J. Adhes. Sci. Technol. 2005;19:817–55. [63] Zhandarov S, Gorbatkina Y, Mäder E. Adhesional pressure as a criterion for interfacial failure in fibrous microcomposites and its determination using a microbond test. Composites Science and Technology 2006;66:2610–28. [64] Hampe A, Kalinka G, Meretz S, Schulz E. An advanced equipment for single-fibre pull-out test designed to monitor the fracture process. Composites 1995;26:40–6. [65] DiFrancia C, Ward TC, Claus RO. The single-fibre pull-out test. 1: Review and interpretation. Composites Part A: Applied Science and Manufacturing 1996;27:597–612. [66] Hampe A, Kalinka G, Meretz S, Schulz E. An advanced equipment for single-fibre pull-out test designed to monitor the fracture process. Composites 1995;26:40–6. [67] Grützmacher H, Wang J, Chiappone A, Roppolo I, Shao F, Fantino E, et al. All-in-One Cellulose Nanocrystals for 3D Printing of Nanocomposite Hydrogels. Angewandte Chemie International Edition 2017; 57, 2353-2356. [68] Fouassier J, Rabek JF. Radiation curing in polymer science and technology. London, New York: Elsevier Applied Science; 1993.

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[69] Lovestead TM, O’Brien AK, Bowman CN. Models of multivinyl free radical photopolymerization kinetics. Journal of Photochemistry and Photobiology A: Chemistry 2003;159:135–43. [70] Ho, Kingsley K C, Lamoriniere S, Kalinka G, Schulz E, Bismarck A. Interfacial behavior between atmospheric-plasma-fluorinated carbon fibers and poly(vinylidene fluoride). J. Colloid Interface Sci. 2007;313:476–84.

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HIGHLIGHTS Glass fiber surfaces with photoactive and methacryl-functional groups are prepared.

•

Single glass fibers are embedded in a photocurable acrylate resin formulation.

•

Surface modification facilitates a distinctive increase in maximum pull-out force.

•

Ductile-type failure of fiber-matrix interface is observed.

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Adhesion of fiber-matrix interface is improved due to covalent bonding.

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Performance of photoactive fibers is superior to passive methacryl-functional ones.

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